Even though the 360 games are dual layer, they are still limited. Each disc has a standard partition on it of about 250 megs of dummy video data too as part of the security. So that is 250mb less of usable space. I can see a game using more than a dual layer disc simply for cutscenes. They might have to go back to the PS1 trick of putting games with a lot of cut scenes on multiple discs.
Microsoft did me and sonny a favor using dual layer discs though because they (and the burning equipment) are readily available and relatively cheap.