I'd actually been meaning to post my thoughts on this issue at some point, but this thread on Maintankadin prompted me. In it, the OP asks "How do you feel about having an assigned Guild MT?" and more specifically, the drama and dissidence it can cause. So, I figured I'd repost my response here.
The only thing worse than having a set MT is having tanks who get bent out of shape over not being the set MT. It's easy to say "Having an MT just causes drama" but that's not quite accurate - having an MT can be a catalyst, but the jealousy of the other tanks is what causes the drama.
I'm in the somewhat uncommon position of being not only a Protection Paladin, but the Raid Leader and Tanking Officer as well, in a progression-focused guild. This resulted in being forced not only to understand the strengths and weaknesses of the Protection Paladin and be the best Prot Paladin I can be, but at the same time to remain as unbiased as possible and put myself and the other tanks (mostly Warriors) in the positions that everyone is best suited for.
On top of that, even beyond simple class comparisons, it's often best for progression purposes to keep people performing similar tasks from encounter to encounter. Picking up and offtanking an add is significantly different than getting a misdirect and spamming your threat rotation for the rest of the fight, so if one of my tanks is really good at performing one of those tasks then I'm quite likely to want to put them in a similar position for the next encounter. This effectively creates a "Main Tank" position even though I never assigned anyone the title; I've got one Prot Warrior who I usually stick on the Big Bad Guy, and then other tanks pick up adds, or hold secondary aggro, or do whatever else the encounter calls for.
I don't think arbitrarily switching up who tanks what is good for progression. One could make the argument that if Tank A isn't there, Tank B should be able to step in and take their position... but either way you've got a tank who hasn't tanked that specific mob before, and it ultimately doesn't matter if Tank A is around or not when it happens, so I'm perfectly content to just let Tank B learn the fight when he or she needs to. Once we had everything on farm I was happy to swap things around a bit just to let people experience different aspects of the encounters, but until that point, speed is a factor, and risking unnecessary wipes due to inexperience or unfamiliarity when you don't need to is silly.
In addition, as a Prot Paladin, I am the best suited tank in the raid for picking up adds or generating burst threat. This means that on a lot of encounters, I don't MT. Even though I'm perfectly capable of filling that position, the other tanks aren't as capable of filling mine.
In short, having an assigned, title-wearing "Main Tank" might not be the best thing ever, but letting yourself get bent out of shape over it is worse.
I'll make mention that we don't prioritize gear to any of our tanks. That's an outdated holdover from pre-BC raiding where you only needed one tank with gear for most of the endgame content, which is no longer the case.