Oh I don't generally play for free, but there have been a few rare occasions where my friends really didn't want to do anything else so I weighed off the certainty of losing my buy-in (about $3.50) versus hanging around a bit more and having some fun. Fun being, pushing chips around until they're gone, maybe fucking someone over because the money's lost anyway.
Usually I decide to spend it on beer instead. ($3.50 equals about two pilsners at that place)
I never really saw the appeal of poker, though. As far as I understand, even if you're strictly smarter than the other people, if they have a littlebit of skill (aka much better than me), you're still going to play a very long time to come out ahead on average? My experience is, you (obviously) hardly ever get the awesome hands they get in the movies, and the whole pokerface/bluffing thing is quite oversold as well.