What is the difference if the first DLC is on the disc or is ready to be downloaded on day 1? Either way you still have to buy it. I tell you if the Assassin's Creed 2 DLC was on the disc and ready to go from day one, I would of bought it. But since it came out too late in my opinion, I never got around to playing that game again.
Well, the AC2 DLC was DLC precisely because the development team realized that they couldn't finish the game on time, and so they ended up cutting those 2 chapters and a few game features (like the ability to replay single missions) so that they could finish it and get a polish run to stomp bugs.
The fact that DLC was an available route just made it an easy way to continue work on those sections of the game post-release so that the content could get into AC2 eventually.