It is an insane amount of work and I do not think that it is wise for Bethesda to vet through mods. Even if Bethesda has the best coding practices, the best coders, etc, platform specific APIs and platform specific quirks will have to be addressed and it'd just bog down the whole development on future games because the smartest (or most competent coding-wise) would have to be used to solve difficult problems for both the game and GECK.
Then, there's the moderating part. It's an insane amount of work that takes away man hours that could be used on future games. Every version of every mod needs to be vetted to prevent modders from putting in malicious code (personally, I'm not sure about the esm/esp modding part but I am guessing that might be a possibility), or copyrighted work (much more important because Bethesda/MS might be sued), or some "undesirable" content (kill children, nvde mods, etc) after the first vetting.
The man hours (money) put into this could be used to make another game, DLCs, or get some writers to do better writing.