They tried the "adding new vehicles" gimmick in Tribes2, and it totally porked over the gameplay balance. Hope it doesn't have negative effects in Halo3.
I'm not a console gamer. I don't like them at all. The interface is far too limiting. A computer with a mouse and a keyboard gives way more precision and control. I only say this to explain the following: I've never played Halo2, but did play Halo1. Halo2 was never released for the PC, that I could find. I'm not totally ignorant of the ways of Halo2, but still only know bits and pieces of it.