It's definitely nice to carry a healer along, I mostly leveled my priest just for that. I've found the paladin to be kinda worthless - can't cast and run, no good aoe heals. I would run a druid (for wild growth and hots) or a priest (circle of healing, holy nova, prayer of mending, etc.) and just set up some decent healing macros. They obviously won't function as well as a real person playing the healer, but it definitely makes a difference.

I'd definitely do the healer with all warlocks though, since you can't really heal yourself.