I prefer to tank ranged mobs with my ranged dps, in heroics at least, and if there's a mixed group of mobs. Takes some of the incoming damage away from the tank. Basically ranged mobs are extremely weak compared to melee mobs, so it doesn't matter what you tank them with.

Nuking down an untanked target can be a very effective method of gaining control of a pull. For example: In Shattered Halls you will want to nuke down the hunters or those casters with fear (unless cc'ed) right after the Legionaire, and not bother too much with tanking them, cause you're already tanking 2-3 melee dudes, and turning your butt to them to run over to the ranged guys will get you crushed.