I have found that good communication with your co-tank to be much more important overall. There is nothing worse than 2 tanks who don't say boo to one another.
Raid leadership really is a collaborative thing, everyone needs to be aware of the situation. But I can see why some raids opt for a set leader, if only to keep vent chatter manageable. Tanks have a certain amount of expectation placed on them, and I think people look on tanks to lead because of past mmo's.
I would say a dps is in a much more ideal position to take charge of a group, but it really comes down to the person. Perhaps tanking classes attract certain kind of people who prefer to take charge (control freaks