Hmm, seems I don't fall into any of these categories.

I design and build web software during the day, then at night and on the weekends I work on my own iOS and Mac apps. I've been offered full-time iOS & Mac design/development jobs but I don't take them for a few reasons, most importantly that I really like my current "real" job and the hardcore web work keeps my skills sharp.

So I'm not the first category because I'm married and do have a life outside work, I'm not the second one because I can easily find a different full-time job but choose to stay in the position I have, and the third just isn't applicable.

I don't really have a position on the pet-projects-as-interview-question topic, but just thought I'd put in my two cents.