Here are some thoughts current as of this posting. This is a complex subject with no clear answers. Thinking has evolved over time but I think the thinking has matured at this point (Dec 2011)
To summarize, the Gitzo Systematic series allows for interchangeable tops, which include the stock flat plate as well as various center columns and a leveler. RRS currently makes their own compatible replacement plates and levelers. The top is held in place with a single very robust bolt that compresses the circular "mount yoke".
A failure could result from either a loose bolt (definitely) or, more speculatively, some minor deviation from perfect spec that causes loss of full contact (or sufficient friction) between the yoke and mating top plate lip.
Some argue that regardless of the quality of the product or the robustness of that yoke bolt, the mere existence of that (very arguably unnecessary) single point of possible failure is unacceptable. I am in that camp. I am very conservative about this but I also know the value of the gear on top of my tripod.
People used Gitzo Systematics for years with no recurring reports of plate separation. My assessment is that no one thought much about this until the most recent generation, for reasons that will become clear.
Reports of plate separation popped up coincident with the new Safe Lock version of the top plate on what is generally (but not always) the "V1" models, for example, on the GT3541LS verses the GT3540LS. The GT3530LS is also a "V1" model, due to peculiarities in the historical Gitzo model nomenclature and their use of "version" as the last digit of the model number. Any Gitzo Systematic with a Safe Lock plate would be included in this group.
(I am not suggesting that the Safe Lock feature is unsafe- it is possible that very slight changes to the machining of the yoke or the lip of the plate itself was the cause, or possibly a short term QC problem, or many other possible speculations- and it would all be wild speculation. But it is a fact that the multiple reports of separation were coincident with early samples of that model series and it was almost immediate. The presence of the Safe Lock upper plate surface merely identifies the era when the problem reports started. It surely is unrelated to this problem)
It is also possible that the specific model era many of us identify with this problem is a mere coincidence. Remember, nothing for certain is known about this potential problem.
I have an older G1410 Series 4 Systematic. I used it for 4 years, at least, before I probably ever even checked that yoke bolt. It was just out of sight and out of mind.
Historically Gitzo has never recommended carrying a loaded tripod over the shoulder. Some other makers of other related products, such as some heads, also do not recommend that practice - when pressed. It seems to me it is a subject most makers want to avoid but when pressed do not warrant their gear for that use. That makes sense because a heavy lens such as a fast telephoto puts tremendous stress on the components in an "unnatural" way, especially when combined with the impact force inherent with the up and down motions from even gentle walking, much less a rapid stride.
Some users of the 'V1' series Gitzos reported loose or less than fully tight yoke bolts out of the box and that was speculated to be the reason for the relative rash of separation reports. It is likely Gitzo responded to that, paying closer attention at the factory. It is also possible that the tripods involved were toyed with at a store and the yoke bolt loosened. We will never know and cannot know the answer to that.
Some users that either reported plate separation or reported the plate "migrating" upward (but not separating) also reported that the yoke was tight and that was something they were very aware of and were careful to tighten the yoke. One of those users is a mod here and I respect his opinion and honesty in his assessment of that. No one will ever know the Ultimate Truth of that aspect of this issue.
Another Nikonian reported success roughing up the mating surfaces of the yoke and mating plate lip. He suggested maybe the machining was "too good", or something similar.
I have not seen plate separation reports for some time now, but I do not crawl the net looking for them either. But I suspect if it were still occurring with any frequency we would hear about it here and elsewhere because any incident is generally widely reported and recirculated.
Naturescapes.net (NSN) came out with an ~$70 add-on plate that eliminates any chance of separation. I have used one since day 1 on my own GT3541LS and I would personally not use a Gitzo Systematic without one because I do carry a 500/4 over my shoulder. I hike my 500 a mile or more on most outings, although I try to use the carrying stance illustrated in my current avatar, which among other benefits relieves lateral stress on the support. However, when working through vine infested areas and other brush it may be necessary to do some sort of over the shoulder carry, or something that puts the same stress on the mount.
Because so many long lens users are now using the NSN solution or something similar, it is likely the "high risk group" has shrunk considerably and that alone may account for the lack of recent reports.
I do run into long lens shooters with the "V1" Systematic models that are using the stock mount and have not had a problem. They are always very surprised to learn of the potential problem. I always show them my NSN plate and give them a brief explanation of the potential problem.
Considering all the above I can only come to the conclusion that I cannot come to any firm conclusion, other than that the inherent design of the systematic is not ideal for over the shoulder carries. There is too much money on top to worry about one-in-a-million lightning strikes. I do not believe enough is known about this problem to attribute it to user error or manufacturing or design defects. I do know that that line is very blurry and it is a fact that the stock systematic is subject to that single point of failure, at best dictated by the tightness of one bolt- that is an accident waiting to happen.
If you are looking for trouble, all tripods have one potential single point of failure, which is the 3/8" bolt connecting the head to the mount. In theory that bolt could shear, resulting in the head plus payload falling to the ground. I'm ignoring that in the following because there is no solution to that potential problem, nor have there been many reports of that bolt failing, except maybe with some very inexpensive tripods. I don't ever recall reading about a Gitzo bolt failing, or the Markins titanium bolt supplied with their Mountaineer replacement plate (TB-20/TB-21/Tb-30).
All tripods with center columns have a potential failure point where the 3/8" head stud screws into the center column. Somewhere in there, there is some glue, epoxy, or some other adhesive holding the female threaded insert in place. Although not commonly reported, I worry about that with my monopod, and would with any center column tripod (including my Series 2 Mountaineer if I had the center column installed). Years ago there was a very widely circulated report of at least one Induro tripod failing in this way.
RRS and Feisol, which both make Systematic type tripods with removable plates, took an approach using 3 grub screws to retain the top plate. There is arguably safety in numbers since it would likely require all 3 screws to loosen before the plate separated and long before that happened the user would or should notice the plate rattling. I have handled both of those maker's products and looked at this issue carefully.
The Markins TH-300 solution for Series 3 Systematics replaces the entire mount and costs $100 more than the NSN solution. For that $100 you get the following benefits:
1. It supports the stock hook, which the NSN plate does not. On the other hand, I have a carabiner looped through my NSN plate and I actually prefer that to the stock hook. That because I can attach small items and do an over the shoulder carry without worrying about things falling off the hook. That is a very personal preference and I can see people preferring the hook over a carabiner.
2. The package is lighter and somewhat cleaner although I think the NSN plate is a clean solution- you would have to look closely to see it.
3. Markins claims improved vibration performance. I have no way to evaluate that, not owning the TH-300. Nor have I seen any attempts yet to do those tests. It would be interesting to do controlled tests but that would require some effort, swapping mounts at least twice.
4. Aside from the bolt issue common to all tripod, there is, in my opinion, no conceivable way for the TH-300 to fail. It is too much robust high grade aluminum.
I believe the Markins Mountaineer replacement plates (TB20/21/30) are similarly safe because although it is a two piece design (it screws onto the mount) the threads involved are too robust to worry about failure and there are far too many turns of the thread to worry about the thing spinning off over time. I have owned one for 7 years now and I have studied this carefully.
Markins also makes the TH-200 for Series 2 Mountaineers, which replaces the mount and the Markins TB-20/21 to create one integrated mount solution. Markins does not currently make a Series 5 replacement so in that case the NSN solution is the only commercial "fix" I am aware of.