I don't just think that DLC can be harmless in some cases, I think it can actively help a game's longevity. If a game was good, you might be willing to put extra money into it for DLC which can aid developers and publishers with the sequel or additional content. This means that they get all the money as opposed to none when a used copy is sold. Take for example Shadow of Mordor, possibly my favourite game of the year - I completed it very quickly, and I'd be really glad to chew through some extra content. If it was free I might be quick to dismiss it, whereas if I pay for it I want *all* of the playability out of it because I put money down for it.
That's not to say that it's perfect. Like M. T. Dremer said, if additional goodies and rewards are being held back to be sold then I too get angry. That's just scum tactics, plain and simple. Destiny has proven that both expansions' content is in the game, the second DLC - which will be sold in 2015 - has been hacked into and proven to exist in the game files. To me that's intolerable, even if it's having extra work done later it's still on the disc and therefore should be playable. So when practices like that exist I'm horrified to think of what other games are holding back.
DLC is great for supporting developers and publishers, as well as getting some extra - or different - types of gameplay from the base game. It's when it's clawed out visciously to sell a quick buck that I have a problem with it.
As I'm guessing you're rather passionate about the subject I recommend you see The Escapist's "Jimquisition", which has a lot of discussion about DLC practices. ^^