In reply to d80f0u:
I have sometimes booked two nights in a hut in the expectation of doing a route and getting back late-ish and needing a bed, then doing the route quicker and deciding to go straight to the valley on the second night, but always phoning or calling in the hut to let them know. Same if book a night or two in advance, then change routes - phone in and cancel. Some huts get fully booked in season, especially weekends, so you would be denying others opportunity.