You wouldn't have to single pull in a dungeon, even if untanked mobs were actually dangerous to squishies. As things stand, as long as you tag everything, you should hold aggro.

I do think there's probably a happy medium somewhere between the occasional aggro loss causing a raid wipe and being able to heal a dps through boss autos. Personally, I think bringing back mob crits and letting the Tank Mastery trait prevent crits on actual tanks is a fair way to do this. It worked in WotLK.