With instant book on, I find it often that guests who wish to extend their stay do so by make a new booking right after their extisting one. These scenarios should be treated as alterations for two reasons:
1. guest shouldn't pay extra cleaning fee - now I'm not complaining if someone wants to give me more money, but it's not fair since i won't be cleaning the room and it might lead to wrong expectations.
2. maximum stay duration setting respected - this is actually the big one. Hosts might have preferential or legal reasons to set a maximum stay duration, but with instant book allowing back-to-back bookings the maximum length becomes irrelevant. Also the review situation gets a bit funny (you get to review people who are still staying).