Where do I start?
I agree that someone is less likely to see a psychiatrist if they sincerely believe that any "voices" they hear are simply angels (or spirits).
I do however take exception to the inference that anyone who believes they have communication from the "other side" is mentally ill.
In the DSM and other official manuals we see set criteria for diagnosis of mental illness. In times past, mental illness has been diagnosed and a person labelled without proper evidence - resulting in inappropriate treatments being given and a general deterioration in the patient due to the misdiagnosis. It is important to state what we really mean by "mental illness".
I will probably regret this, given the environment I'm in, but let me use myself as an example. I function in life very well. I am not plagued by destructive thoughts or "voices" and my life is totally normal. However there are times when I choose to participate in a spiritual service, during which I often experience phenomena which could be labelled "supernatural".
I am totally aware of arguments which indicate that someone is using self hypnosis in this scenario. Whether that be the case or not isn't really under debate. What is under debate is whether I'm mentally ill or not. The answer is that I am not, yet I experience (at my will) things which could be labelled as spiritual experiences.
I have seen spirit beings and I have had communication from spirit beings. Again, these are controlled (by me) and are never disturbing to me. Interestingly, many of these communications reveal information which I couldn't have known and which can be verified - either by someone present or later as things transpire.
I would love science to eventually find a way to measure and prove spirit communication. Experiments have been conducted which show a significant result with regards to extra-sensory communication. These experiments are currently being replicated by those with funding. Many experiments have already shown a significant result beyond chance on things which you appear to be labelling "evidence of mental illness".
Mental illnesses where the individual hears voices and sees visions exist of course - I have studied various mental illnesses and their causes. This does not mean, however, that every individual who experiences extrasensory perception is mentally ill.