Actually ground force holds the car to the ground, keeping it from becoming a temporary and uncontrolled plane. Ground force is your friend, it's one of the interesting battles of racing, the higher (lighter) a vehicle rides the faster it will go, but the lower (heavier) it rides the more control you have over it. Race cars now generate ground force that is multiples of their actual weight, this is necessary at 200 MPH to avoid tragic accidents, so 5 pounds here or there just isn't going to make that much of a difference.
Yeah when you're dealing with the level of horsepower of your average street vehicle 5 pounds can make a difference, 1 HP plus or minus is big. These are race cars, they scoff at the HP of our street vehicles. And of course in bike racing you're dealing with tiny tiny vehicles, many of which weigh less than the driver so obviously the weight of the driver matters much more since you're talking about a significant fraction of the overall weight of the vehicle. 50 pounds is not a significant fraction of the overall weight of an IRL or NASCAR car.
Most of the racing cars and bikes are able to hit and maintain top speeds that are pretty close to each other. But it's being able to "out-brake" the competition (being able to wait longer before starting to brake), and out-accellerate, coming out of the turns that will win races. And that's where the weight reduction comes in very handy.
Mark