Everything is bigger in XMP 'cause we ain't no wimpy 2k4 players!
Switching driver/gunner without exiting the vehicle would be bad. Like someone said, the Jugg would be unstoppable (fire a shot, move Jugg to avoid incoming rocket/shocklance bolt, shoot again, move to avoid second volley, repeat). And can you imagine a Raptor driver switching to the gun before the Raptor even stops moving?
On new vehicles: wasn't someone a long time ago working on a vehicle based on the 4-wheeled V10 Viper engine-powered motorcycle? That could be fun, a rediculously fast but fragile 1-person vehicle.