Zombies serve a purpose in that they allow developers to present a different AI:
They don't have to act like human NPCs, who should - in theory - be able to 'think' about the combat situation and react in a way that makes sense as fellow humans. I thought FarCry was pretty good at this. Not perfect but a decent enough example.
They don't have to act like the typical alien enemy, technologically or biologically advanced compared to the humans we play.
Instead, you get an overwhelmed-by-numbers AI.
As for playing zombie games, I've only played SPAZ and those might as well be aliens or humans since the combat AI is pretty much the same.