The main point of co-op games is to play with friends. In order to do that, you (usually) need to connect to the games official servers, which you can't do on cracked games (for the most part) which makes DRM pretty pointless. It prevents you from enjoying maybe a lazily thrown together campaign and it's more of an insult than anything else.

Plus, it's also worth mentioning DRM hurts paying customers by causing reduced performance in-game and also inconveniencing them.