Do mods and UGC sell games? If done well, it's probably a decent factor. Skyrim and the Fallouts and especially NWN probably would've had much fewer sales if they weren't easily moddable. But it's probably less profitable than we would like to think. I wonder how many people play Skyrim without mods vs. with mods, and how many would've bought it regardless of whether there were mods or not? Anyone have any data on this?
My personal opinion:
I finished skyrim once without any mod. The game is so huge that I will not play it again most likely. There are so many other good games (good = better than skyrim, like D:OS
)
I have never used any mods that change the look of the game or any game mechanics. For me, a good story is better than graphics and some old games (like chrono trigger) are still much better than new games with super 3D graphics (better = fun to play).
I use fan patches ( solving bugs) and fan translations (removing spelling errors, like in Pillars of Eternity where I also reported bugs in the forum).
I have created one and a half RPG maker games myself (more a collection of insane ideas than a good game), I played some rpg maker games and I would play a fan made campaign for D:OS. But modding would not be an importand selling point for me.
Prof. Dr. Dr. Mad S. Tist 
