Does your guild have a main rogue or a main hunter? Silly question, of course not. You might have a class leader for every class, but something like a main rogue simply doesn't exist. But then most guilds have a main tank. Which leads to the question of why you would have a main of one class, but not of all the others. Having a main tank who gets priority over other players in tanking loot distribution is common, but it doesn't always work out that good. Or to quote a comment from Yunkndatwunk from yesterday: "Some of the examples cited are just poor planning: for instance when a whole guild gets together to equip an MT, instead of treating the MT just like everyone else. Then they are shocked! shocked! when the person they treat like a prima donna starts acting like one. It's one thing if it's someone you've known for years, but with most guilds composed of strangers it's just not wise to give so much up front and then hope the person pays you back. People might think that's slower or not progression minded enough, but the other side is you're not progressing when the "emo tank" leaves because you're not babying him anymore like you did last month."
So why do we have main tanks in the first place? Fact is that in a raid of 25 people not everyone is of equal importance. There are quite a number of people who could die without wiping the whole raid. But if the one person who is holding the aggro of the boss mob dies, the main tank, then it is usually a wipe for everyone. Thus if you are looking at things short term, or you naively believe that the main tank will be with your guild forever, piling up the best equipment on this one guy makes perfect sense. Epic tanking gear is the most difficult gear to get in this game, as unlike dps gear you can't get a head-start by using PvP gear. If the progress of the guild as a whole depends very much on the quality of the gear of one or two main tanks, giving them priority on loot advances the guild faster. Everyone is happy - until the day the MT leaves and the guild finds that the not-main tanks have far, far inferior gear. Suddenly the bosses you could still kill easily last week wipe your raid repeatedly this week, raid progress stalls, and other people start to leave to join other guilds which are still advancing. Whole guilds have collapses like that, or at least suffered several months worth of setback.
Adding to the problem is that nobody takes "extra" tanks on a raid. If for some reason a raid needs two warlocks to overcome specific encounters, that won't stop a third and fourth warlock to be invited. But if a raid needs two tanks, the raid leader will invite EXACTLY two tanks, and never three. The extra warlock is useful for his dps, the extra tank isn't, because his dps is so pathetic. Thus for a guild to manage some sort of rotation to equip lets say five tanks equally well is extremely difficult.
Nevertheless I think that for mid-level raiding guilds, where the risk of the best equipped people leaving for a top guild is relatively high, introducing a tank rotation would make sense. Yes, it's complicated. Yes, overall raid progress will be somewhat slower than if you put all the tanking gear on one guy. But the huge advantage would be far greater long-term stability and an "insurance" against setbacks caused by one person leaving. The setback a guild currently experiences if their best equipped "main" rogue leaves is small and easily overcome. If only guilds could organize their tanks the same way!