I think that add-ons are fine so long as they aren't on excess and they don't feel like they were intentionally cut from the game simply to be sold later (for instance, Soul Calibur 5 and their weekly DLC, not to mention the enormous amount of music DLC they have available). Then, there are map packs. I hate, HATE the way map packs are handled, especially in Call of Duty's case (though I don't like CoD, anyway). First, developers have the gull to tell you how much you will be paying for map packs altogether (not all, but some do) and expect some sort of fanfare. You know what's strange? They actually give it. Most of the so-called "hardcore gamers" clamor for this exorbitant money maker. What's worse? They buy all of the map packs when they come out, too. It's sickening, really. Then there are costume and character creation packs, which is a big thing for games like Soul Calibur and Dragon's Dogma. The problem? There are people that me that love character creation and buy them. Granted, I've never bought one, but it's a clever and exorbitant scheme that I can't stand.
Seriously, there's so much more I could go on about, but my point is that I think it can be a good idea (especially if it's free), but most of the time it's just a way to make the player pay for something that's incredibly easy/cheap to make. I like the idea of addons but I don't think it's handled correctly.