I use a druid (boomkin) / pally / lock team and enjoy it. I have the option to use a priest as well since I have one in the 60s but don't unless I really need the healing. I think the druid adds more utility w the various dots / debuffs / and burst dmg spells plus the moonkin buff for the warlock too. Also, in tight spots I like having innervate and treants and when I'm AEing the druid is useful too (of course hurricane or whatever is limited to outdoor use only). If for some reason my paladin goes down I also have the choice of going bearform and finishing off the mob or rezzing my pally back up again. I've found the druid isn't as useful when I need him for strictly healing like in a tough instance and thats when I'll choose to bring the priest instead. Anyway, hope that helps