During the past year, I've actually started trying this out. I've taken a couple of my vacation weeks and spread them out - instead of taking one week vacation, I've taken Fridays off for five weeks. It's been amazing. Somehow it feels like that extra day counts twice: your weekend gets one day longer, and your work week gets one day shorter!
But as someone without kids, being able to take short flights or trips every week over 3 days would be awesome.
I guess ideally, since 5x6h and 4x8h are only two hours apart, a utopian workplace would offer its employees either option -- the people who chose 4x8h would make 2h more wages weekly.
If I wanted, I could work 4x10 (With some sweet talking to a manager perhaps). I'd very much prefer to have 30, 32 or 35h to distribute over the week though, it would make the planning a lot easier. In that case I'd do e.g. 5x6 now that my small children needs early pickup, but I'd go to 4x8 when I have larger children who can bike home from school, so we can go on a weekend trip with friday off.
Of course, if everyone works 4 days a week and has the same 5th day off that advantage goes away I suppose.
Now that I have kids and work an office job, I would much prefer a 5x6 to a 4x8.
Why not? You'd get to spend more contiguous time together.
It does mean the week is more compacted and there's less time spent "twirling your thumbs", but it's a much better schedule over all and we continue to produce great work.
That extra day off would do far more for people than one or two less hours every day. So 4x8=32 hours, vs 6x5=30 hours. I would even bargain for 4 days/9 hours, or 4x9=36 hours.
That third day is huge. It's the difference from having only enough time for some leisurely weekend activities, to having enough time to take a mini-3-day-vacation every week (short flights, etc.).