Seems to me the bottom line here is the critical difference between "why" and "what," i.e., understanding why someone did something vs. excusing it. If somebody commits murder and the defense calls experts who testify that the killer is this, that, or the other, fine. Now we understand more about why it happened. But it doesn't affect guilt or innocence, and with very few exceptions, it shouldn't change what the punishment should be.

So taking the air marshal incident as an example, the guy's actions - regardless of the cause - make the marshals' actions reasonable and justifiable. If the guy is indeed bi-polar, we have a better understanding of why it happened and can have more empathy for him and his family and friends than we'd have if he was an extremist bent on murdering innocent people.

I also think "off" is well, off in stating that people make up conditions or mental illnesses to explain their actions. The conditions are real, what some people do is hijack them as an excuse for what they've done. The fact that mental illnesses are used as inappropriate excuses doesn't mean they don't exist.