I've been a happy AirBnb guest on a few occasions; the one time I had someone's whole usual studio apartment I found it a bit strange how much of their life was left on display for me – like the host's prescriptions still in the fridge! – but of course respected their privacy.
Each time I've been a guest, we've seen photos of each other on the site, and the keys were handed off in person. While I didn't expect anyone to stop by and check on me during my stays, I always had the impression the host or host's friends were nearby.
A guest with criminal intent would try to pick a place for a longer stay, in a more anonymous building, with a host known to be out-of-town. But then again, that's also what someone seeking to burglarize any vacation-emptied residence would do. More or less this same sort of crime could happen without AirBnb, or perhaps be enabled by nothing more than tracking public tweets/'check-ins' to predict unit vacancy.
(Another development I eventually expect in this progression of tech-mediated sharing: a bad-faith host who surveils their guests.)