I think you might be making a fundamental assumption that quality of each hour of the day is the same.
Maybe for a particular business it does not matter, they just need an employee on the clock to cover time that customers may come in, but I think many businesses have tried this out, and found that they get about as much from their employees in 32 hours as they do in 40 hours.
I think it helps to give people time to rest and deal with life so that they can be more focused and thoughtful when they are with you.