If anything, they will "verify" somehow that the first installation of a disc is, in fact, the first. And you can use it without the physical media in your drive. The next installations that are not on consoles tied to your account, will require the disc.
This "feature" would almost work like SC's spawn system, and you can loan your game to a friend to play.
The requirement that it is installed is so they don't have to take 50gb of data on an 8x (guessing on this one) blu-ray drive. That would be loud, slow, and annoying all around.