I'd rather use couchsurfing.org. Not only is it free, but the host verification seems better.
That being said, Airbnb.com is a MUCH slicker website, and most consumers are more comfortable with a professional-looking site and with services they've paid for.
I tried using couchsurfing on a recent trip to Japan - the interface was horrible and I got no responses. I ended up using AirBnB and found a super cheap place in the nicest neighborhood of Tokyo with an awesome host who took the time to show me around (and visit a semi-legit hostess bar.)
Not everyone is able to get a room where they want when they want, of the quality they want on couchsurfing. For many possible hosts, money can make it worth their while to open their doors to a broader variety of visitors, and perhaps support the opening of nicer rooms to visitors.
I'm a couchsurfing afficionado as well, but this site appeals to me, partially because it ISN'T free. I always feel too obliged to my hosts, and handing over a small some of money would assuage my guilt that I hadn't thanked them enough.
That being said, Airbnb.com is a MUCH slicker website, and most consumers are more comfortable with a professional-looking site and with services they've paid for.