- last post: 01.01.0001 12:00 AM PDT
The Elites are meant to be bipedial, so they can wield the weapons. Without that, they wouldn't serve much purpose.
Grunts, I assume, might run on all fours for two reasons: either they're adapted for being quadrupeds, which is very possible (considering their limbs' lengths), or their methane tanks are a pain in the back and they run on fours to make it less painful.