So if you make centaur cavalier it's one whole package! Sure, gaining mount isn't relevant but do cavalier's charge and other similar powers keep working for him?
Personally, I would say yes. One of my players is using a centaur right now, and I'm simply going to count him as being mounted and having the mounted combat feat for the purposes using trample, ride-by attack, and spirited charge. It just just an additional class feature he gets in trade for his level adjustment.
By raw not ofcourse, but I don't see a problem with it if it doesnt destroy balance in the party.
By raw he could get an exotic saddle and ride a dire bear. Then we all just back away very slowly.
I'd let them benefit from everything except the ride check to prevent damage. Cause that's silly.