I used to ask this question, but now I've generalized it to asking if someone has any hobbies. I find passion in any area to a good indicator of the type of people I want to hire.
I think this is far better than "pet projects". Some people would rather socialize, read literature, play music, play or watch sports, work on cars/home improvement, paint, travel, camp, write, get involved in politics, read the news, or any number of things! Rest and relaxation are particularly helpful to mental health. Even playing video games, watching movies (with a certain amount of moderation), or doing things that are more classically endorphin-releasing "fun" than programming contribute to a well-balanced individual.
I'd rather hire someone with a sense of balance in their life.
Further, its also discriminatory against people with families, or other hobbies, or against people who don't want to do the same thing that they did 40-50 hours during the week on the weekends too.
I'm not saying that parenting isn't time consuming but it's not as good an excuse when it's so obvious that your lack of time for more programming is also caused by much less noble activities than parenting.
Of course people are going to watch some TV with their families, or play an hour of games to unwind a little, or go watch some sports game with their buddies, or spend god knows how many hours practicing their musical instruments. This is what keeps people sane, productive, balanced, and happy.
Having other hobbies after work is generally a good sign that the person is mature, and has good perspective on life.