Originally posted by XStAnt
The shows only have a 10 or 15 minute window because they start at scheduled times. The FP return time is designed to get you there right about when they start seating people for the show. This way they can make sure all FP holders get into the show.
Check the schedule of shows that you get at the park. If the end of your FP time is still about 10 minutes before the show starts I am fairly certain you can still use it. They will have already started letting in the standby guests by then.
If they gave you a 1 hour return window for shows you could show up right when a show has started. You'd then have to wait for that show to end (20-30 minutes) and then wait however long it is until they start letting people in for the next show. That defeats the idea of FP and would make people very angry.