Downloadable content is just that -- extra content for the game. It's not part of the game, it's not a necessary addition or patch. It's just extra stuff. Given that context, it's reasonable for any developer to make anything for their game and charge a few bucks for it. Nobody is forcing you to download new skins or new models or new missions, it's just there.
Shivering Isles and Knights of the Nine, both Oblivion DLC, were awesome. They added a whole bunch to the story and to the game. They weren't required, and plenty of people never picked them up. That's okay. It was just there. This was coming at a time where, trust me, better and bigger things were being made by the mod community.