Collections have their value, but they are somewhat "heavyweight" (eg they take up a lot of UI space).
For many purposes (eg only two or three movies in a series - think eg Get Shorty, then Be Cool) what would probably work better is the sort of lightweight grouping offered by some other library apps, where you group the two or three movies together, and they appear as a single unit in the primary UI (perhaps with a slightly modified poster art that looks like a stack of two or three items).
Obviously this requires a little thought as to display purposes (in which views do you display the separate movies, in which do you display the stack?) but I don't think this is an impossible problem. Start with the heuristic of "if there's one movie that fits the view, display it; if two or more, display the stack" and work from there...
I'm not criticizing collections - for large groupings they are great. But they take up so much space, and are simply not a good practical (and quick to create...) way to tie together just two or three movies.