It's probably the opposite - the marginal value of the work done from 32 to 40 hours is a lot lower than the first 32. In fact I doubt most organisations would be able to distinguish between the output of someone working 32 vs 40 hours. Instead of trying to measure though it seems fairer to just keep a straight hourly rate no matter how many hours you work.
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.
