Conceded, it can be a slippery slope for some. For me it's not, thankfully. I've no doubt the rigid clock-on/clock-off process helps to keep some people from overworking, but that isn't me. Even in the office I would show up when I finally got to showing up, and would leave when I felt I'd, personally, done enough for the day. I have absolutely no qualms with putting down my laptop when I feel I'm done.

The major difference is that instead of my mindless scrolling through HN or something at 2pm being on work time that is just lost to unproductivity, it's now not because I can get up and do something (grocery run, make lunch, etc.), and conversely that burst of focus and energy in the evening now goes towards productive work rather than focusing more on a videogame or something.