GV collisions + lag = very bad and strange things. Flying apart at ludicrous speeds, or becoming welded together like that one X Files episode where the Aurora pilot got melded with a rock.
There was a bried period when collisions were enabled (or functioned because of some bug) in AH2 beta. I ran into someone (Kev maybe?) and I as drove into his tank with mine, his rolled over on it's top and there was some other funkiness. Definitely not something I'd like to see in game.