More than 600k people uploaded their files and about half of them had already played the game for more than 100 hours (that was when the game was out for only three months). That was almost uniformly in single player mode, as almost all of the 600k files showed that time spent in multiplayer was 10 hours or less.

BTW, the quests sometimes flesh out things you encounter in the main quest, tie up loose ends and continue some characters' stories.

But back on topic, whether I like to pay for something is not so much dependent on what it is, but more on if I think it's worth paying for. If people dedicate time and resources to create something I enjoy, I gladly pay, and if only to make them keep on creating stuff.

As soon as money is involved, competition for it starts and that is usually a Good Thing(tm).

Funnily enough, SE giving away these quests for free is a way for them to make money. People who completed the main game are not as likely to sell it as SE promised to make new stuff available every week for one year. That means late comers do not have access to a sprawling used-game market, and may buy a new copy. Obviously SE makes money from sales of new copies, but not from sales of used games.

So in the end, somebody paid for these quests, too.