Having played Oblivion on PC, I can safely say that user-generated mods saved that game. Fixed the AI, improved the graphics, that sort of thing. Same goes for their last few games as well. But what of Bethesda's next game, Fallout 3? Things are less certain. Pete Hines:

Folk probably took for granted that every time we make a game, there's a mod tool. We explained to folk that it takes a lot of time and effort to get that tool ready for release, and it's not on our schedule right now. We need to get the game done and out. It's not to say we won't do it. It's that right now we have an enormous amount of work to do, for three platforms and all these different languages to get it out around the wall.

Sad to hear. Maybe modders can step in and get mods working? Interview: Pete Hines On Fallout 3 Mods [Rock, Paper, Shotgun]