I propose it because it would help to ensure that the credit card wasn't stolen (because the cardholder would spot the charge before the guest arrived). If you can ensure that the card isn't stolen, then AirBNB has a way to track down criminal guests, which makes it less likely criminals would see this as an exploitable system.

Yes, it's a pain, hopefully they can come up with something better. But they seem to be ignoring the problem of effectively anonymous guests.