We have a warrior tank in the raid, currently i'm the only one. But we have 3 rogues, 2 Dks, 1 or 2 rets. And rest is ranged. So the tank can do banner and rally.
Rogues get spots all the time. I don't think I've seen a rogue get sat at all thru tier 15. If they did it was rare.
So melee spots are limited. Then it also comes down to which classes take the least damage. If you're bringing in 7 heals the last thing you want is someone that takes a lot of damage. Rogues are really good at avoiding damage.