It's really all about team dynamics and corporate culture.

See, successful entrepreneurs are, more often than not, binary beings that totally devote themselves to their company and expect other the other people in the team to do so as well. Someone can come and work for 5 hours and accomplish just the same as he would over 11 hours but it may send the wrong message to the group - that the company is secondary to personal life and that just meeting your obligations is really enough, no need to do beyond that.

Furthermore, some people wouldn't be able to accomplish the same over a short working day; how would that make them feel spending 11 hour days in the office, while their colleague leaves after 6 hours and still gets appreciated?

Hopefully like an unproductive lump who needs to learn how to turn off Hacker News and Facebook during the day so they can also get some work done.

So providing he doesn't burn out doing it, it is probably going to help the company a lot if he can put in the extra hours to be across some of the lower level stuff.