I just turned IB off after seeing this post from a host.
A 24 hour time lag is needed so that the host has time to cancel, and if they do the guest does not get the address and private information.
This host had an IB and cancelled it, the cancellation was confirmed by Airbnb - but the information had already been sent to the guest as soon as his reservation was made and when she got home he was in her apartment.
Quote from post: "Airbnb did send a confirmation for the cancellation. The problem is as soon as the potential guest payment goes through they are given and address. That is the dangerous part of this instant booking."