We're harmless to others in this state... too harmless, which is why we've actively worked towards not doing it. Dissociated, we're extremely passive and docile; we look to others for direction because we know we're too crazy to trust our own judgment.
This is good, because it means other people can tell us not to jump off a bridge. But enterprising abusers can take advantage, and they have. They do something relatively minor to space us out, then do something major, and when we surface from the episode, pretend it never happened or that we misunderstand what transpired. And since they're the sane ones and we're crazy, obviously our judgment can't be trusted over theirs. They won't even have to cover their tracks, because dissociation distorts our memory. (For bonus points, add that when the memory does become clear, they can claim, "Well, it didn't bother you BEFORE, so obviously you're just trying to make me feel bad now, and it's too late to do anything anyway.") Dissociation is very handy around inescapable trauma; when it comes to abusers, though, it can keep the relationship going, because we can't learn from what we don't remember.
If you're a dissociative, and you notice that you're dissociating a lot more around someone, or find that your memory becomes increasingly scrambled and unreliable around them, that is something to be concerned about. You should not constantly need your coping mechanisms in a healthy relationship. Sure, hard times happen everywhere, but there's a difference between a stressful time and a stressful PERSON. Even if that person is not manipulating or harming you, you might just plain be bad for each other.