Yesterday, I had the same question : I went from 5 hunters to 4 and decided to include a healer for the instances. Its a lowbie team so far. As I have already a 5 shaman team and another 5 box team with a priest and also a duo rogue-druid, I eventually settled for a holy paladin.

I like the auras and the buffs for the pets, the wisdom for the hunters. Another difference with another healer is there is no fade or equivalent, so my pala can take aggro but he can also take a beating compared to a clothie. So, I have a new follow key where only the hunters followed my main hunter, leave the pala heal himself, take some distance to shoot the mobs. The pala can be a kind of bait for adds.