I have:
1x Prot Paladin.
2x Elemental Shaman
1x Dual Spec Elemental/Resto Shaman
1x Destruction Warlock

When an instance is hard, and you are relatively undergeared..
The Resto is a nice option, to make things much easier.
But it is also slower to finish or kill things.

3x Elemental, no Resto... can heal all 5-man content with ease.
Or 4x, if your team is Pally/Shammies.