It's my understanding that when people say "working hard" in these discussions, they aren't comparing 25 hrs/week vs. 50 hrs/week. They're usually comparing 50 hrs/week vs. 75 hrs/week.

If I forked my life into a version of me that worked 50 hour work weeks and one that worked 75 hour work weeks, the 50-hour me would win, hands down. The 75 hour work week me would outright miss most of the key strategic decisions that got me where I am. When I look at everything I've done that in hindsight has been a huge career boost - getting involved in the Harry Potter fandom, making friends on the C2 Wiki that got me a job in financial software, learning Lisp & functional programming, founding a startup, and getting a job at Google - they all happened in the downtime between work. Had I simply worked 75 hour work weeks since 2001, I would be a physics grad student right now, hating it, and making a pittance. I'd probably be a damned good physics grad student, but that doesn't help me very much.

Now, I also put in quite a few hours into developing skills and building a track record, and I don't think those opportunities would've opened up if I hadn't. But I wouldn't have thought to look for the opportunities if I did nothing but concentrate on work. You're a big fan of necessary but not sufficient conditions, right? The work is necessary, but so is the downtime.