Bethesda does not "rely" on modding to finish games. Can you play to the end of the game without mods? Yes? Then the game is done. Could it be better? Hell yes, it could be better... a more precise way to pose the original question is to ask if
PC users rely on modders to "finish" the game. The answer is "yes"... yes we do. Unofficial patches, leveling fixes, new/better textures and meshes, gameplay mods addressing specific concerns like enhanced vampires/werewolves... the list will go on and on, and after DLing a few of these, the gameplay experience for a PC user might be drastically different from those who play on the console or un-modded. This happens with nearly every game that can be modded.
I fully plan on modding the living hell out of Skyrim when the CK hits... there are so many game mechanics that could use tweaking -- I have quite a long list in front of me. But even after saying that, I would be lying if I also claimed that Bethesda
relies on the modding community to finish their games. The fact is that they have provided us with as much content as they possibly could have in the timeframe they were given. Creating and balancing something on the scope of Skyrim was no easy task, and why are we surprised that there are places here and there that could use a little polish? How many millions of playtesters suddenly got on board after November 11? Can they afford to playtest the final product with as many man-hours as all of us have put in?
It's time to get realistic here. The Skyrim that I am playing now is not the slapshod work of lazy devs who spent most of their time chomping pizza and laughing about how much "free" work they will get out of modders like me. They did a good job... they really did. But we can help make it even better, and I'm thankful that Bethesda has the balls to release a CK at all... a lot of software companies jealously guard their intellectual property instead of holding open the doors to their code and making powerful modding tools for us to use. I can think of a lot of games that could have benefited from a modding community and dev tools, but they were never to happen. This is one of the reasons that the TES franchise has such a loyal following after all these years. Y'all wouldn't be posting in this forum if you didn't care.

-Loth