Since you mention it, I think the only genre that benefits from DLC is the music and rhythm genre.
Adding a couple of songs adds to the game's life and the possibilities are endless. The DLC in other games, though, are mostly superficial, shallow extras which could have easily been added during development.
You can use the same logic for music games; they could have just added more songs during development...
But really, you can't just keep adding stuff to a game before releasing it. Eventually you have to call it done and ship it. Additional content can be fun even for games that don't feel lacking in content on their own. Back in the day, it seemed like every successful game got at least one expansion pack adding new missions and perhaps a few new features. DLC has the potential of providing that same quick "more of the same" more economically than doing a full retail release.
In fact, if you take the concept to the extreme, you get episodic content, which a number of developers have been praising for a while (though TellTale is the only company I know of who's really gotten the quick release schedule down).
Basically I'm saying DLC has the potential to be more than just "superficial, shallow extras" for genres other than music/rhythm.