I’d think lowering the truck would decrease its usefulness. Pickup trucks are designed for rough terrain, but you can’t take on rough terrain if you’re in a low-rider. Plus, it makes the truck look stupid.
Lifting a truck, which most rednecks down here in the South like to do, seems to increase its usefulness in rough terrain, like @Cruiser said. Plus, it supposedly looks more “macho” lifted. And according to some country friends I have, it’s easier to hit mud holes with a lift kit – and who doesn’t want to hit mud holes, right?~
Also, most people lift their truck for the purpose of accommodating bigger tires. (Though, I say bigger tires usually means smaller…well, you know.) The bigger the tires, the worse your gas mileage. That doesn’t decrease the truck’s usefulness, though. It just makes driving it more expensive. However, with bigger tires on a truck that wasn’t meant for them, the axleshafts, bearings, brakes, and steering gear will all wear out faster.