I fully agree with you that a DM has to be *very* careful with powerful PC. However, power =/= leadership.
In my game one of the DM's "PCs" at the moment is basically a demi god. (it's not D&D but whatever). If he gets mad, he can level a city (but he tends to kill *everyone* so the PCs really don't want that to happen). However he is letting the party lead him around (it's part of a quest) because he knows he can't trust himself in a fight (due to berserk-ness) and in general due to faulty memory. Even though he's immensely powerful, he's a terrible choice for a leader and he knows it.
Let's look at lord of the ring. Gimli was a dangerous warrior, but he wasn't the leader. But let's say he wasn't just a very good fighter, but the *best* of the land. Would he have lead the party? No, it wasn't in his character/nature to be a leader, no matter how good he is at swinging an axe...