Sometimes, there’s a point in a discussion – perhaps not a totally agreeable one – where the other party utters those words. It’s usually a tricky thing to deal with.
On the surface of it, it’s a recognition of human fallibility; a concession to an inevitable condition of being human. While it’s obvious no one would seriously contend they’re anything but, the sort of person who makes this statement explicitly is usually the sort who typically acts as if they believe exactly the opposite. They only concede the reality as a tactic, because no plausible self-justification occurs right at the moment and they need time to think. Because it’s sort of a given anyway, it “costs” them nothing.
In one sense, it’s a perfect stratagem – only a boor would “pile on” after that admission – fair play demands you not kick someone (further, or at all) when they’re down. The unstated premise of the admission is that, having said what they said, the party’s inviting you to admit your own imperfection. In other words, the confession becomes a lever for reciprocation.
You should resist this temptation. Here’s why: Conceding “I’m imperfect, too” trivializes whatever it was you’re disagreeing about to the point of making it entirely beside the point. In other words, the general condition of both players will supersede the specific problem or incident at issue. It’s not purely a question of “winning” an argument, rather the avoidance of getting sidetracked by an irrelevancy. One possible response might be, “Does that mean you’re sorry?” If they are, everyone can afford to be magnanimous, and simply forgive.
If they’re not sorry, you can point out the speciousness of rationalizing bad behavior by pointing to their imperfection but not accepting any responsibility for that behavior (I have criminal clients who do this all the time).
There are no absolute rules for discourse. But it is handy to understand when someone is using baffle gab, evasion and smoke to avoid personal responsibility.