For one thing, I keep having to deal with the appalling ignorance of a large number of people, because they all think like that--and that, if I'm mentally ill, than I'm also mentally defective. (Which, arguably, I technically am. But you know what I mean.) There are in fact several mental illnesses strongly correlated with high intelligence and/or creativity, but that doesn't stop people from making unwarranted assumptions about me. I mean, you don't have to think I'm a genius or anything, but I think we can all agree that I'm not stupid.
For another, fairly or not, concerns about someone's mental health can put a serious damper on things like their hiring prospects, whether they're true or not. This is, in fact, one of the reasons I'm so outspoken about my own mental illness. It's important to me that the average person knows that, yes, you can be mentally ill and not the way "everyone knows mentally ill people are."
And, for a third, mentally ill people aren't the way a lot of people assume mentally ill people are. Yes, some of them act the way certain CTs do, but the majority of them don't. Further, the majority of CTs don't act in a way comparable to any mental illness. (Except that I'm given to understand that extreme conspiracism is actually going to be listed as a disorder unto itself in the DSM-V. But think about it--would they have to do that if it fit a previous diagnosis?) I have known one or two and known of one or two others who do seem, to my untrained perspective, to fit the diagnosis of paranoid schizophrenia. However, because I am not an expert, you should take even my above-layman's perspective with a grain of salt.
And finally, it isn't like expressing an opinion of anything other than medical issues. Because mental illness is a medical issue.