I've thought about this for a long time, and think it'd be a bad idea. First, the obvious, less working hours mean less pay. Which then means some people will be forced to do OT just to reach 40 hours make ends. Even further, some people will be doing 2x OT to make up any traditional OT they were making. People will be so burnt out that the extra day wont mean shit.
3 or even 4 day weekends people would look forward to due to holidays would be null and void.
Also, if we go from a 5 day to a 4 day work week, I can easily see people getting an inch but trying to take a mile by making the work week even less.
That's just my take though. I could be completely wrong and that shit changes the work mindset for the better.