Personally, part of my extracurricular time is occupied with activities which actually make me more valuable, such as open source work, organizing study groups,

Personally, part of my extracurricular time is occupied with activities which actually make me more valuable, such as open source work, organizing study groups, and presenting at meetups. But I also have family obligations that I cannot set aside, and I can't do all of these things without burning myself out. Fortunately I am in a position where I can afford to sacrifice some compensation — and that's what I choose to do. So even setting aside your point about the marginal value of the last 8 hours (which rings true), in my case there is definitely no question about the value of each hour.