ODesk enforces the use of their time tracking software for hourly projects. Their staff economist, John Joseph-Horton, has some interesting papers about the justification for this. He argues it is actually in the interests of freelancers to have such a system in place.
Basically, freelancers prefer to be paid hourly (or daily, or whatever), rather than per-project, because clients love ratcheting and requesting endless tweaks otherwise. But clients prefer to pay per-project, so they can predict how much stuff will cost and not have to risk paying for freelancers that pad hours. ODesk's solution removes some of the risk for hourly billing, making it a more acceptable solution for clients, which works out for the benefit of freelancers.
A similar concept is how companies benefit from being able to be sued. Because you can sue them if they screw you around, you feel more comfortable dealing with them.