It depends.
DLC that is developed alongside the game, but withheld from being apart of the final product, is a cheap way to make more money. And if it's already on the disc, then that's an even bigger slap in the face. However, DLC offers a new way for game creators to add to the game after it is released. Developers can create extra bonus missions that may not have anything to do with the plot, but are still fun. DLC also let's developers go crazy with ideas that could never fit in the main game, but are still fun. This way, people who buy the game can still get the full product, but people who love the game have a way of extending it.
It would be impractical to say that there should be no DLC at all: not even DLC made after the game has been out for awhile. Not wanting to pay full price for an unfinished product is completely understandable. But expecting developers to just give away extra content is ridiculous.