See http://www.spaceflightnow.com/shuttle/sts114/050716ecographic/
“…to make sure the ship’s engines don’t run too long, draining the tank dry with potentially catastrophic results, after other problems that might prevent an on-time shutdown.”
Now, WHY this can be catastrophic, I did not find the physics behind it (yet). But a “funny” story why you always want to be able to rely on your fuel gauges popped up. Enjoy:
http://www.damninteresting.com/?p=744