I have used and hosted extensively on couchsurfing, but before the AirBNB days. I don't think the community is quite as thriving as it once was, but yes it is actually free.
If it was convenient, if they seemed interesting, respectful, etc.
It's a system largely based on reputation and feedback. Being unwashed, having a lot of stuff, not ever hosting, or having negative feedback will throw up red flags for hosts.
(Not saying that homeless people are necessarily like my description, but I imagine those would be factors in being rejected).