Many people will play an engineer when they buy a vehicle. If the vehicle gets damaged they can hop out in a "safe" place and repair the vehicle themselves. Then after repairing it they get back in and go fight again.

Well some people aren't very smart about where they get out and they don't pay attention to what's around them. About four or five times now I've snuck up on the engy and either killed him first or stolen the vehicle and run him over. When you see an engy repairing a vehicle be sure to look at the faction symbol for the vehicle itself. If it's the circle with a line through it instead of GDI or Nod symbol, then the vehicle is empty and you can steal it.

It's great fun driving around in captured vehicles because most people look at the vehicle and not the symbol or name of person driving it and you can run over enemies who think you're on their side.