Are main tanks a mistake?

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 f