I have to work 40,000 hours in my life, roughly, everyone has a number. I can spread it out, 3 day workweek, or get it done with. If I work 5 day weeks, banking 2 days pay, the target gets hit earlier. Banked money grows at 7%, it's like getting raises of 7% on the money, that's rough math. If you can't afford to bank 2 days pay/week how could you work a 3 day week, don't argue about the feasibility of a 40% savings rate.
Overall I find the MMM accomplishes a similar outcome with less overall days worked. His way front loads the benefits though.