Third, an update set takes advantage of the renewed following/buzz for a movie when the DVD is released. Everybody's back in the mood, and it's valuable free advertising when you have a card set available.
As for Cindy's Point #2, I'm sure they have some costume and prop material set aside for MM3, 3D3, SDCC, and future compilation sets. They haven't used the "Harry Potter Connections" idea with characters, though there's a similar thought in 3D2. (But it's true that they wouldn't have made arrangements with the studio to hold out extra materials because they didn't have the license yet.)
There has to be an idea for a set to celebrate the opening of the Universal theme park. (Like the Literary set, that might be a separate license. But the mind boggarts with speculation on what might show up in the gift shop at the park!)
