Self righteous people who think they know it all might assume that. Reasonable people will think "that person experienced something I don't understand".

> we do people a disservice by agreeing with their self-deception

We do people a disservice by assuming we know more about them than they know about themselves.

I don't claim or believe I know it all, but I try to support any claims of knowledge with evidence.

> We do people a disservice by assuming we know more about them than they know about themselves.

Someone who knows more about human biology or human psychology than me may, in fact, know something more about myself than I do.

I didn't say you should accept their claim at face value. I said you should not assume that the cause the the experience is mental illness, but rather accept that they had an experience that you don't understand. Was it god? Was it some sort of neuralogical event? Was it the onset of paranoid delusions? You don't know and assuming you do is arrogant.

> Someone who knows more about human biology or human psychology than me may, in fact, know something more about myself than I do.

Just because someone is an expert, or has more knowledge, doesn't mean they can't be wrong. When you dismiss someone's subjective experience because you think you know better (rather than just taking it with a grain of salt) you do them and yourself a disservice.

No, they can be wrong in their explanations of the cause of their subjective experiences, but they cannot be wrong about the experience. People who feel the presence of the Holy Spirit really do feel something. It's just that what they feel is most likely not caused by the Holy Spirit.

> we do people a disservice by agreeing with their self-deception.

And we do ourselves a disservice by dismissing spiritual experiences as mere delusion. See:

http://lesswrong.com/lw/n9y/is_spirituality_irrational/

particularly the resulting discussion.