1. I think it will ruin the RPG part of the game. WOW and all the other MMOs can pretend to be RPGs as much as they want to, but they never really are. If you go into any MMO, you will never really get to role-play because there are specific "builds" that you are supposed to use. For example if you wanted to play a rouge that had turned to shaman magic, and had shaman gear, you would not get invited to raids because you did not "build" you character they way you were supposed to.
2. The added other players would take away the freedom in the game. In Elder Scrolls the thing I like most is the freedom you have to do whatever you want to. If there were other players you would not be able to do this because of preserving the "game balance." I think it would also take away the freedom to make your own class, and also take away all the mods.
3. It would dumb down the game. Making a MMO would bring in a lot of players new to TES, and therefore they would most likely dumb down the game with designated "starting areas" in which you can not do anything interesting.
These are just my reasons. Does anyone else have any comments on the subject?