I fully agree with the last paragraph. When it comes to valuing my time, the less free time I have, the more valuable it is, and the less reluctant I am to spend it (the value of an hour of my time isn't constant).

If I'm busy at a given time, it might not take much for me to go out of my way to help a friend; but if I'm really busy and they ask, there had better be some kind of incentive.

...upon reflection, that would be why people get paid time and a half for overtime.