It's not a necessary evil by any stretch of the imagination. It is an evil that is cultivated by the shitty atmosphere surrounding games and their development that produces not-quite-there products that have to be finished after they are released. Basically, DLC is a very easy way to have customers pay more for content, in the sense that virtually all paid DLC ever made for large games has not been equal, on a dollar for dollar basis, to the amount of content you get in the vanilla game. Look at Dark Souls. For the sake of argument, you paid $60 for it. The PTDE DLC is $15. Is it about another quarter of the game? Fuck no. It is one of the better DLCs ever made, and the content is great, but no way is it equal to a quarter of the vanilla game.
There's also the fact that many things in games that we take for granted could easily be cut out as DLC and not impact the quality of the game that much, but appear to be a massive improvement as DLC. For example, can you imagine Dragon's Dogma with no buffing magic? I certainly can, and it wouldn't be a blatant hole in the game. Or Dark Souls without the Path of the Dragon covenant. Half the players don't find it anyway, but it would be the s*** if it was DLC afterwards.
Then there's pay-to-win or games that handicap themselves to mandate purchasing DLC, but all that sort of disingenuous crap is widely despised anyway. Premium ammo in World of Tanks.
Some DLC is done well, even though there are almost universally more expensive than the same amount of content in-game. These basically amount to expansion packs. The only things I hold them to here is that they not be integral to the main game, that they not be available at launch (my opinion in a nutshell: only the vanilla game should be available at launch, any extra content is an effort to make people feel like they're not getting the complete game, which is disingenuous) and that they not be cut from the main game, which can rarely be proved.
I was going to go on for a bit more but who even cares. I've said all I need to.