Games have their flaws. Anything made by humans are destined to be flawed, as humans are flawed by their own nature. Some of those flaws can be easily (OK, sometimes not so easily) with a patch. It is usually done against flaws of technical nature, like glitches and unbalances in gameplay. But how do you fix flaws who comes with the very nature of the game? How do you fix a boring combat or repetitive gameplay?
Some try to do it with DLC, by adding variety or new game modes to the games, trying to answer problems the gamers have found in the game. So, some DLC can be much appreciated change. Except if the game is already not that good, like problematic controls or uninspired game play, there is very few the DLC can do to save it.
And them you have the problem of asking gamers to fork money to have this DLC who can improve the game they already have. And that is hardly a good thing. Asking more money to improve a game is not exactly a good business practice. DLC must not be about correcting mistakes in game design, but to add more to an existing game.
Developers are now in the pressure to finish their games before a finish line so the game can launch in a certain date. Honestly, I prefer them to take more time polishing and putting more in their games than rushing the development process. I think every gamer would be happy to have a better game later than a not so good game now. Or at least it should be that way.
So, in the end, I think DLC is not the right way to improve a lackluster game. It is better to spend more time in development and launch a better game than trying to fix it later.