There's a rather large fuel pump that relies on the fuel gauge to know when to shut off. If it doesn't shut off when the fuel supply is empty, the pump can do bad things. I don't know what, exactly, but overheating and burning up would be a good guess.
Dave Stoller's question is more pertinent. Why the shuttle indeed? There was actually a good commentary on NPR a few days back talking about the shuttle is moving away from NASA's original goal of space exploration. All it seems to do nowadays is ferry stuff to the Int'l Space Station (aka black hole for money) and fix the occassional satellite.