For me personally it's more about variety. I love bdubz vehicle mod for that. I don't have to build the same bicycle, minibike, motorcycle, 4x4, or gyro over and over. I can build a humvee, or a golfcart, or whatever.
Also kind of funny that the motorcycle is better at offroading than the 4x4 is... you'd think an offroad vehicle would be better and driving offroad.