Depends on how it's implemented. Borderlands 2, for instance, had additional campaigns and characters, which exist independently of the main campaign (largely due to the fact that your interaction with other characters is consistent regardless of who you play as). Assassin's Creed 2, by contrast, was very obnoxious about it. It literally includes a point in the main story where it points out that there's missing content, pulling you into the animus loading room just to tell you that 'this memory is locked for some reason' and that you have to skip it. I don't mind the Borderlands example, but the Assassin's Creed 2 one really rubs me the wrong way.