He's a tech guy, first and foremost. He's been dedicated to releasing the source to his games, because he's dedicated to sharing the tech that went in to these games. To the point that he went back and made changes to the Doom 3 source[1], at the time a four year old game, to appease the lawyers who were shy to allow them to release it because of software patent concerns.
If you're looking for the game, then yes the source is useless to you. If you're looking to learn what went in to the game to make it work, the source is literally invaluable. With art assets, it would be better. But by itself it is such a precious treasure. No one else in the industry is releasing the code to their AAA multi million dollar budget games. There are not enough words to describe the value of the contributions Carmack has dedicated to open source and release.
A colleague and close friend of mine directly attributes his early programming days to the fact that he was able to take the Quake 2 source and hack away on it. He's now a programmer in the games industry. You just cannot put a value on that kind of knowledge and examples and source being freely available. It sings to the core tenant of what Free as in Freedom is all about.
[1]http://www.tomshardware.com/news/DOOM-source-code-depth-fail...