>Most Manhattan buildings are large and have many units. If you rent out your apartment a few times a year, how is this fair to your neighbors, who pay the same rent and never agreed to have a bunch of transient guests in the building?

Do you personally get to vet all your neighbors? No you don't, and the results in pretty much no difference in how things typically play out. If anything, anyone who runs a respectable AirBnB has more information to go on rather than a simple question of will they pay rent.