Five days in four - what's so wrong with that?

Wanted to follow up on Damian Wild’s observations on KPMG’s invitation to staff to switch to a four day week. We now know that something like four fifths of the firm’s partners have stepped forward to cut a day out of their working week.
Nice to see them leading by example. I suppose they are hoping that in doing so other staff will be emboldened to come forward to do likewise rather than sit in fear of volunteering being seen as a lack of commitment to the firm.
However, some commentators on this site (scroll down to the bottom of the story here) have pointed out that you have to be very careful in cutting days, especially for people who would normally work long hours anyway.
For example, let’s say the working week is 40 hours – eight hours a day. Could that still be done in four days? It certainly could, I’ve done it myself, which means there’s a risk of a culture emerging that assumes the same hours will still be put in no matter how many days are on your contract.
Of course, conscientious individuals might chose to do that anyway. And they are free to do so. But that’s very different from management making the assumption.
Some might argue that long hours are assumed anyway so, what’s new? Well, it’s about being up front with your employees and it’s about being straight with them that a cut in days means a cut in days – not just shifting it somewhere else.
Now, I don’t say that KPMG will do that. I merely highlight the risks involved in changing working practices.