I dropped to 4 days from a full time job and my MIL echoed what some of the pp are saying... when does my husband get a day off, why does he have to work full time without a break etc etc.
My husband doesn't do anywhere near the amount of housework/cooking/washing/shopping/admin that I do, so when he finishes work, his evenings are free, mine are not.
Having a day off didn't affect my pay too much when you take into account we then didn't need breakfast/after school club on that day.
Having a day off meant I could book all doctors, dentists, bank, solicitor, etc, appointments without having to take time off work.
Getting on my errands and housework, laundry, batch cooking done on my day off, meant that our weekends and evenings were now completely free and we had much better quality time together as a family.
The only benefit to me was that one every couple of months I was able to get a hairdresser appointment! And even that was more for my DH as it meant he didn't have to mind them on a Saturday for me to go!
So I may have been off my paid work, but it was to make it easier to deal with the mental load and do house/family work, without me running about like a blue arsed fly every evening and weekend.
My MIL would only have had a point if all housework/child-related jobs were split 50/50. Which they weren't, I was doing them all while working the same amount as my DH. His evenings and weekends were always free - I've just leveled the playing field by making sure I now have an equal amount of free time, while only actually costing the household "family money" roughly £20/week.
(Now A SAHM after 3rd DC born and a poorly DM)