Saying that everything that's a DLC "should" have been in the original game is nonsensical.
The DLC is everything that they couldn't get done in time. It's DLC because they couldn't get it in the game. The only way to get it in the game would have been to push back the release date, but you can't keep doing that forever. The game has to come out eventually. So, eventually there's gonna be DLC.
Its silly to think they are 'extras' because its highly possible that they were ideas that were thought up and created during the development but purposely left out so they can stick a price on it and expect people to cough more money up in order to get the complete packages. Its business sense and its bonkers.
This is just false, and allow me again to http://www.google.com/#hl=en&sugexp=ldymls&xhr=t&q=Conspiracy+theories&cp=11&pf=p&sclient=psy&safe=off&source=hp&aq=0&aqi=&aql=&oq=Conspiracy+&pbx=1&bav=on.2,or.r_gc.r_pw.&fp=a34f7f1872f21971&biw=1280&bih=887
It's entirely possible that DLC content is mentioned during the development. However, games are on a schedule. They have to come out eventually, so time constraints means that now and then, a dev will have to work on either A or B. You can't always have both. So what happens? The developers choose to work on project A, because it's important like the combat system or something, and leave B for later because it's just a fancy house or a side quest. What if they never get to B at all? Would you rather just never see that content, and let the ideas just vanish? Or would you rather they finish it up after the fact and offer it to you as a DLC?
They are not fully developing an idea, then sticking it on a flash drive somewhere to release as DLC "later." No, if they develop something before the game comes out, then it's in the game.
DLC does not take anything away from the game. All it does is allow you to add more.