I have done this with my priest all the way 70-80 and am doing the same with tank paladin (now 44).
On the plus side, if you play your main spec once you get to 85 you should know it inside out and _should_ know what is expected from you to face any instance, heroic or raid (skill wise, not gear wise).
On the minus side, it's boring as hell to do the same stuff over and over. When you're low level, and you have heirlooms, levels fly by so there is some variety. 70-80 (I've done it before Cata) it's a pain.
Why do I do it then? Well first because the idea of leveling thru zangarmarsh, borean tundra or (add any zone) for the 7th time is giving me the creeps (more than repeating instances anyway), secondly because after years of raiding and stuff I stepped back to casual play, so immediate queues (tank or healer) + full rested xp almost always + mobs in instances giving lots of xp is a far better scenario than questing as far as I'm concerned.
Edited, Apr 7th 2011 10:45am by lostraniero