Heuristics are, unfortunately, quite valuable in hiring scenarios, especially for start-ups.
It's also just much more fun to work with people who love programming, and an easy way to tell (even if it's susceptible to a few false negatives like your friend) is to find out if they program in their own time.
The only exception to this was a guy who did amazing work on side projects, but the work he did for his paying job suffered significantly...
The thing is though, you can love programming and find that 40 hours a week of it is more than enough. I think what you'd find with this heuristic is that it tends to favor young people. It's a lot easier to be enthusiastic about some side project when you're 20 than when you're 40 and have a lot of responsibilities.