Blu-Ray discs are big. Most games not developed by Hideo Kojima only take up a fraction of the disc space. So that space is going to waste! And can be better occupied with movies.
Sony's director of hardware marketing, John Koller, has said that later this year, the way certain PS3 titles - like, say, movie tie-ins - are marketed and presented is in for a change:
We are actively pushing, and the way that we see the future is that the movie and the game are placed on the same disc. There are a lot of developers who say, we have this game based on a movie, wouldn't it be great to marry these concepts? We will definitely see this stuff this year.
It's sure to cut down on packaging costs, but will that in turn increase the cost to consumers? Crucially, Koller didn't say.