You're probably right, if I did have kids I would prefer 5x6h until I shed that responsibility (they leave the house once adults).
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.
I do 40 flexible hours already, so I do a couple of hours in the evening or on the weekend, just to get a 6-7h typical day, to fit the schedule with school pickup etc.
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.
Having a 4 day work week would mean one day off without kids (they are in school/sitter/day-care) which would be great and would give you tons more freedom just to get things done. Either chores around the house, need to take the car to the shop, visit the DMV, any number of things that are extremely difficult and busy on a Saturday with kids.
Of course, if everyone works 4 days a week and has the same 5th day off that advantage goes away I suppose.
