I've thought about this before. Some people will be likely to try to "milk the contract" by taking more time to deliver than is needed. Besides being necessary to deal honestly with one's clients, I strongly think it's in my self-interest to avoid such milking. Instead, I try to provide as much value as possible per hour, and if I can increase efficiency I'll be better able to negotiate higher rates at some point down the road.

(Besides the fact that there's no shortage of software work, so it would be really hard to run out of things to do no matter how efficient you are.)