the cam shaft sensor basicly tells the computer what cylinder is near tdc, so it knows when to fire that injector. without it your truck will get bad gas mileage.
if it was a bad crank sensor, then your buddys are right, your truck would not start. the computer would have no idea where the pistons were in the engine.
sorry if it is kinda vauge(sp), but i just woke up and have a hard enough time remembering their purpose when i am awake.