halo's ghost is exactly what i was think for handling, but faster top speed.
as far as justifying the need of the role of bikes, it kinda depends on how the quads handle. i feel like they might not have a fast enough top speed. i would really like to be able to reach insane speeds on a bike, and i think that would add an incredibly intense and visceral experience.
picture the speed bikes from return of the jedi. you know you want that. i dont think the quads can deliver that.
edit: just an idea i'd like to throw out there. inspired by the tron bikes
give the bikes a kind of plasma based caltrops to drop behind them in a trail that they can use to damage armored vehicles when they run (or hover) over. imagine chasing down another biker while shooting your forward mounted (low damage) gun, and they let out a trail (think tron bikes) and cuts you off. it would give the bikes a role vs larger vehicles and a game to play against other bikers that is very fast paced and skill based.