Don't you wish sometimes that mobs had to play by the same rules as players? On his WoW
blog, Gitr relates the story
of how his tankadin came upon a group of 4 Gordunni shaman in Feralas and had to suffer the combined effects of their melee and spell attacks, while out-damaging their healing as well. Undoubtedly, part of the problem is that, as a paladin, his stun options with Seal of Light up are almost nill -- it's simply a product of the class. But the larger question, and the question that Gitr explores at modest length, is why aren't NPC spells interrupted by damage like players are?
Obviously, there are balance issues to consider here -- if damage interrupted mob spell casting, quick-hitters like rogues and warriors would have an even greater advantage over casters than they already do. Gitr argues that giving casters the ability to interrupt casting by hitting mobs with their own spells would add an added utility to faster, lower-damage spells. While I generally agree in principle with anything that gives squishies more survivability, I think it betrays the balance between casters and melee. In general, melee have an advantage over caster mobs and casters have an advantage over melee mobs. That paladins, and one specced for tanking at that, have problems downing healing mobs is no surprise -- it's a function of the spec. Still, it's an interesting discussion to mull over, even if I don't quite agree.