> My complaint is about some small group claiming to have a monopoly on a normal human faculty, in this case rationality.

"Rationalists" don't claim a monopoly any more than Psychiatry does.

> The small group might well go on to claim that people outside the group lack rationality.

Again, something that psychiatry is quite noteworthy about: the entire point of the profession is to tell non-professionals that they're doing Emotionally Healthy wrong.

> The mental health profession do not claim to be immune from mental illness themselves,

Rationalist don't claim to be immune to irrationality, and this is in fact repeatedly emphasized: numerous cornerstone articles are about "wow, I really fucked up at this Rationality thing", including articles by Eliezer.

> they do not claim that people outside their circle are mentally unhealthy

... what?

So if I go to a psychiatrist, you think they're gonna say I'm FINE? No matter what?

Have you ever heard of "involuntary commitment"?

> and they do not claim that their particular treatment is necessary for mental health.

Again, this is about as true as it is for rationalists.