Necessary? Nah, they already have enough variety in classes and specs and shit as it is. 'Fast melee guy' is already filled several times over for DPS and 'lol i can heal myself guys' is a DK thing. They added it because it looked cool and this was the perfect expansion to do it. Can you imagine if they didn't? It would be a wasted opportunity.
Shit doesn't have to be necessary, it just needs to fit the expansion, be cool and not break the game. The not breaking the game part is not even strictly to do with new shit, it's to do with any balance changes or updates but is most obvious with entirely new things.