Mods are great!
I think mods are one of the best invetions of the video game industry. Gamers already feel as if they are a part of the game world that they enter each time they play, and there is a strong desire to mold that world into something more of your own. Mods allow gamers to do that.
I like how Dr. Jerz also mentioned that it keeps gamers interested, which is why companies allow them I think. If I were a game designer, I would be a little apprehensive of letting gamers create their own levels or maps because of those copyright issues that Dr. Jerz talked about. I think that keeping them interested though outweighs that apprehension. Gamers want control and like the ability to change the game to what they would like to play most. Mods are what's going to keep those gamers returning to that old game sitting in the cabinet that is out-dated compared to newer purchases.