From Strategyinformer.com: "Strategy Informer: You've mentioned how the story missions are going to provide unique experiences not possible in the main game. Is there a reason for that? Can the technology not handle it?
James Torbit: It's more to do with tying those events into the storyline. There's two halves here - the first half is the open world gameplay, bunch of toys in a box, the other half is the story line and these moments that you're going to have on the missions. We don't want them to be arbitrary, and many of them are very complex. Some of these are very difficult to build especially outside of the context of a mission, and they are technically difficult to achieve. But these are moments we want so we keep them in the game."