I agree with this but on the flip side you at least get a bit of a heads up before they come (if anyone chooses to let them)
If they end up blocked (and it would be pretty much equivalent to blocking as who would let them stay?) then you end up with people booking with new acounts, or on friends accounts ect, and it becomes an issue when it's a 'surprise' awful guest.
Potential for damage/ less caution on the side of host maybe.
Again i agree that its not ok for crappy guests to be able to just book, but i'm also concerened about making sure airbnb block people better.
If a host had 2.5 stars they'd be suspended, how is a guest still able to even look at booking??