For my part I've been doing a mix of freelance and salaried work in the last few years (as a data scientist), and I have reached the conclusion that a strict 40-hour week is not an easy thing to handle. I think it stems from the fact that the "knowledge worker"/hacker ethos is often deeply rooted in a self-driven and motivated agenda of learning and experimenting with new things on a constant basis, very often just for the sake of it. This can easily conflict with a regular work schedule in terms of number of hours and commitment. But the point is that it really shouldn't, because very often, those two "modes" nourish each other, which can result in a stronger, more robust and ultimately more meaningful and happier work like.
I've been trying to convince my current boss that a 3-day week would be a more efficient and compressed use of my time, while giving me more time to pursue other contracts and projects. But it seems that there are some cultural barriers that makes it a difficult message to pass. I actually intend to use that thread to show that I'm not the only one in that situation, that such a culture really exists, and thus that it must be taken into account somehow.
It might be somewhat cliché, but I really believe that this mentality is a glimpse of the future in terms of work ethics.