I wish regular employers would adopt this more as well. My value is what I do/produce, not how much time I spent doing it. Some days I produce massive amounts of value, while others I barely get anything valuable done. I'd like a work place that would encourage me to go home and enjoy myself on the bad days and stay as long as I want on the good days, instead of expecting a regular 40 hour work week every week (I have flex, but on average...).
On the other hand, I guess my employer has this issue too. We usually have an upper (money or time) limit on a contract but charge by the hour. And this is pretty much the standard in my industry and a requirement from our clients. How did we end up this way? It makes no sense!