Doesn't it depend on the situation? As someone who struggles with a couple of issues (which could be worse, as my family history is a hot bed of mental problems. At least I don't have severe social anxiety, epilepsy, ADHD, and am not bi-polar! Seriously, the vast majority of people in my family have been certified for these things and more. So I am relatively normal!), I never bring up my behavior as a 'excuse'. I will make it into a joke with people who notice (such as my obsessive touching every one of my finger tips in order until they 'feel' right), or mention it when giving people support if they are suffering from the same issues. In my blog I haven't mentioned my issues yet, and if I do make a post it will be to raise awareness rather than to excuse dickish behavior (because that shit don't fly).

If they are actually using it to excuse their behavior, then that is bad. Now, if they are using it as a explanation in conjunction with a apology then I don't see the bad in that, as long as it is in the form of 'it's not you, it's me'. I have had to apologize in the past for some of the things I did because of my own problems, and I will usually mention them- not to excuse or get out of anything, but to explain that it is my problem on this and I will try to do better.

So yeah. I am giving you the benefit of the doubt in that you are only talking about people who use it as a excuse. If you mean it otherwise, then saying 'no one should mention their issues ever' is frankly unrealistic and feeding into shame culture. Because people aren't perfect, and sometimes they like to get things off their chest. *shrugs*