Depends, really. I'm sure a lot of day one DLC is stuff that was held back just so they could sell it separately. But in some instances it's something that didn't fit in with the original game or couldn't be completed in time, so I suppose it's better to give people the opportunity to buy more content for a game they love rather than for the content to never see the light of day.
What really pisses me off is how mindless some publishers or developers are in their attempts to squeeze more money out of a game. From what I understand, the BioShock 2 map pack was a huge clusterfuck because 2K didn't want to create a separate playlist and split the community. So unless you happen to end up in a lobby in which every single person has splashed out for the DLC, you'll never actually get to play on the maps (outside of private matches, I assume). It just seems hugely inconsiderate of the most devoted BioShock 2 multiplayer fans... and let's face it, there can't be that many of them.
