Gundato: While I find some 0Day DLC to be questionable, you also have to keep in mind that it isn't (always) just a case of chopping things out. After a certain point, the art and level design people aren't needed anymore, since the game is being beta tested and patched. So rather than sit on their hands (or go work on a different game), they can work on more content.
Its usually done a shitload earlier than most people think. Referring to the attached image of the project development phases, it'd be pretty rare for a development studio to make major changes such as adding locations/quests/characters after the design phase. The implementation phase is just making what was planned.
A good developer will forcibly lock down the requirements before the implementation stage and ruthlessly stick to it no matter how many awesome ideas are generated in the meantime. This prevents scope creep which slowly expands the project until it's twice its original size and has to still keep within the original budget & timeframe leading to a shitty end product that tries to do too much and fails it all or doesn't deliver what was promised. Discipline is the key to project success.
As an example of the opposite, look at the development of duke nukem forever. That is a textbook/horror story example of a project gone crazy with enthusiasm and scope... well I was going to say scope creep but the sheer amount of movement was such that if you could call
that creep, you could call the blitzkrieg "german creep".
In a well managed project, any good ideas thought up after the requirements were locked down could be stored away and developed in parallel when the people are available to work on it. With the delay caused by the platform owner approval process (at least as far as consoles go) and for duplication and distribution, its entirely possible for these extra addons to be ready at the same time that the main game is released.
All that said, paying to unlock content thats already on the full priced disc is "teh ghey" and should be punishable by a molten glass enema