When I hosted (2008), we were very active hosts that summer and we basically required people to write personalised messages telling us who they are and why they like the sound of staying with us specifically. We got a lot of generic copy and paste messages and we ignored them, since we got so many messages. For about 3 months, we were hosting people almost every single day, often multiple people (and the most at once was 12, for a weekend while there was an event on).
This worked very well and the only "bad" experience we ever had was one of those 12 people that stayed the weekend of that event was.. harmless, but a bit unpleasant and odd, and that was because we relaxed our vetting a small bit for that event.
And honestly, I don't know if I would have paid for an account at that time, as an active host. I got into it because my housemates were into it and when we were planning on moving in together, we discussed it. If it had cost money to join, I can't imagine we would have bothered, certainly not all of 5 of us.
With that said, nowadays, I think some barrier to entry would make me feel more comfortable with it. Back then, in my early twenties, I was a bit more relaxed about sharing my personal space, I guess.