So in either of those extremely drawn out posts did anyone really explain WHY you would choose to bring thief over another class to nyzul? In this day and age where 99's are just a days abyssea burn away and you can get af+2 while just doing an empy or other classes gear; there's no real reason to bring a 'subpar job'.
The problem comes at the content, not the class(so much). Thief isn't where it could(and arguablly) should be, but the problem comes more to the mentality of the population and the content. Nyzul for instance is mainly a slaughter fest at the fastest possible speed; which means heavy DD's and minimal support. Thief, regardless of skill level, lacks the DD ability of 'average' heavy DD's[especially outside of abyssea].
Obviously I'm not saying NEVER EVER NAHNAHBOOBOOO bring THF to nyzul(pre-abyssea I brought my thief to every nyzul[because of my group make up]) and we did just as good as the next guy. The problem extends to the population, after determining how the event will be played. While a good THF may not extremely increase your chances of failboating; the chance of doing so is greater because of how the class plays. In the sense that in order for X thief to be 'on par' with Ydd it requires this gear and a lot of 'skill'. While all Ydd has to do is melee in one set of gear and occasionally hit a ws macro.
The discrepancy in skill level NEEDED to be effective is so large that many people forming a PUG would much rather look for classes that require less 'hope' they won't be bad(in this case war or sam). Wether people know they do this or not, I'm not certain; but if I'm forming a pug(for whatever reason) I go the same route.