RE: Revenant as a class in GW2 - There is a reason why it wasn't in the original release

Lore-wise, yes Revenant is basically just a Shaman mixed with Necromancer.
As for why you'd want to play one... it'd probably because they're the jack of all trades in heavy armor (with Elementalist being the light armor one, and Engineer being the medium). They have great damage options, great support options, and great control options. Similarly, they can do quite a bit of different things within a single build and can even specialize in quite a bit of options at the same time (ie. they're not just a second rank X because their ability to do Y is nearly on par with X despite having more options). Their trade off is that they have an involved energy mechanism for using effects, but that largely isn't that difficult to figure out on a pretty solid level (and really is something that makes them more appealing if anything, as being different is nice).