If you have it legal paid thru iTunes, then Siri should find it and you can play it via iTunes in the cloud or you can search thru that, which renders that issue moot.
I'm guessing you don't buy much from iTunes. Or you haven't been for very long.
At the drop of a licensing hat, any one of your movies can be removed from the catalog. Sometimes it will come back, but somehow, not as the same movie, not as the one you 'bought'.
So, while you may have bought SuperAwesomeFlick in 2011, you don't actually own the SuperAwesomeFlick that's currently on the iTunes store (even though, they are the identical film version). As a result, if you delete your copy, you cannot redownload it. Nor can you stream it from the cloud on any device. It is only available if you sync it manually (homesharing).
Of course Siri is not going to help you play these types of movies, it will only direct you to the current version on the iTunes store (which you don't actually own). And obviously for the movies and TV shows that simply disappear from the iTunes catalog and never come back, Siri is of no use either.
It's not a lot to ask to want an interface that lets you browse movies (especially those you own) without hassle. Plex, Netflix, Infuse all have interface schemes which far surpass the current implementation under Computers (even as bad as Netflix and Infuse currently are on the ATV).