Knowing that Tripwire doesn't really have a PayPal donations page or something, I see cosmetic DLC as just a donation with a little extra thrown in as an incentive. That being said, my buying these DLC is just to show my support for TW to continue on with KF-basically a donation. TW devs need to make a living. It's not like they're as big as Activision, Ubisoft, EA or any of the other big names out there, and they push with free content regardless. By selling cosmetic DLC's with free event content, it's mostly just to offset the costs of development (I'm guessing), and it's not like the payment is forced upon you.
And if you have the mentality that these DLC's are just donations for the company, then you should be happy. You should expect little to nothing back on donations, but TW is awarding you with a complimentary skin as a thank you for donating. And remember, it's not like cosmetics affect how the game is to be played.
Character skins are like Payday player masks: you don't even see these aesthetic changes yourself but it gives you the reputation of accomplishing something for these changes (in this case, donating to TW). But the weapon skins are a bit different. You (as the player) can actually see them, as well as other players in the same game.
If TW changed the names of the DLC packs to something like: "Donation Pack 1 (Outbreak)" and "Donation Pack 2 (Nightfall)" then perhaps people's perspectives of these DLC's may change, but ofc we may never know.
Anyhow, this is all just personal perspective.