Face-melting

[Dasschlechte]

Is a facemelt (shadow) priest different enough from a warlock to justify having both? And if the only reason is healing, is it different enough from druid healing to justify having both?

Although having my own healer would be nice, that druid belongs to my mother (seriously)

[Perfects]

Yes, you can have both.
Druid healing is mostly HoT's while Priest healing is more direct healing / AoE healing.

Shadow priest is also different from warlocks. As a shadow priest you need to manage mana better, and you have different spells, that makes the class different.. Besides, not all warlocks are affliction and have a lot of DoT's.