This is an evasion I have grown tired of when people are defending someone's moral behavior, especially a politicians. Yes, of course we know the person in question is not perfect, but that is not the standard we are holding them to.
A woman at my previous church said something like this while defending Bill Clinton in the late 90s. "Oh, we want our leaders to be perfect. It's like when people were so upset with Jimmy Carter said that he lusted after women in his heart in the 1970s." I recalled that pretty well, and my strong impression was that Jimmy was being criticised for campaigning by being interviewed for Playboy, not because he lusted in his heart. It seemed one more convenient rewriting of history. We weren't asking Bill to be perfect. Any amount of lusting in his heart we would have taken in stride. It was lying to - oh, I'm sorry, misleading a Grand Jury that had conservatives frothing, and even the general public who showed up at the controversy mostly for the sexy stories were upset that he had taken power advantage of a young woman, that they had tried to destroy her reputation, that he had flat-out lied to us on TV, that he had done this in the Oval Office, and that more darkly, it looked like he had treated other women worse than that in the past, using intimidation, violence, maybe even rape.
This on top of cattle futures, legal records from Arkansas showing up inexplicably in the White House, using FBI files for political purposes, and all the rest, month after month. No, we weren't asking him to be "perfect." Just a whole lot better than he was.
I hear that "not perfect" excuse even on Great Books podcasts about authors. I am not asking that Jean-Jacques Rousseau be regarded as perfect. I am asking that his abandonment of his children to a foundling hospital not be glossed over when discussion the moral sermons he was giving the rest of us on how society should be structured. The weak excuse is something of a reverse strawman, implying that it is the accusers who are being hypocritical and applying an impossible standard.
The excuse can be used reasonably, which is why people try to steal it to apply unreasonably. One might say of a boss who pays well, doesn't play favorites, keeps business going and people employed even in hard times, but loses his temper and yells at times that he is "not perfect," if they are advising someone whether to work for him or not. But to use it when he is engaging in criminal acts, or threatening, is to stretch that excuse too far.
If you catch yourself using it, quickly reflect whether you are using it justly or as an excuse.