Would a game like Resistance: Fall of Man which was their showcase title been not possible on a dvd9 disc?
So you're talking about the Blu-Ray storage advantage here. Resistance supports many different languages and 7.1 audio, cutting this out of the game the game and downgrading the sound to max 5.1 would probably fit on a DVD. The point is developers don't need to make such sacrifices on the PS3. Killzone 2 will also support 20 languages and they could use 50GB discs if there's not enough space left for other data, like high quality textures. (Note the PGR4 downgrade, complaints from Rockstar with regard to DVD limitations)
Blu-Ray also offers the advantage of supporting Blu-Ray movies, default anti-scratching coating and sustained data reading speeds across the disc (predictability).