Some might say that's a stupid question. Of course nothing good can come from an illness.
This is the sort of thinking which psychopathology promotes. The term means the study of illness of the mind. It's sort of like negative thinking in that it sees only the negative side of the labels.
Depression for example. It is the clinicisation of misery. It has a negative prognosis. It causes social disability. It can lead to suicide.
It can also lead to self-reflection. It can be part of a change process. It is associated with critical thinking, the very quality which makes research psychiatrists good at their job. Certain types of intelligence are boosted by depression. Learning can also happen and it's my guess that people like Stephen Fry have so much knowledge because when they're low they may read more.
There are people who value schizophrenia. Martin Luther King was one. He saw its value for creativity. There are areas of employment where people with bipolar can be successful, for example in the media. Even psychopaths can flourish as lobbyists or entrepreneurs.
The mentally ill have also contributed to humanity from Jesus and other religious figures to world-changers like Lincoln and Churchill (both voice hearers). These people may have lived in times or cultures where their gift could be valued. Our society may not allow gifted people like van Gogh or Jean D'Arc to prosper as they did and contribute to the human race.
This doesn't mean the mentally ill should suffer or be allowed to suffer for the value of the experience. The acceptance of the suffering is a choice by those willing to see the experience for what it is. The pain is not something to be wrought on any person nor any person allowed to suffer when they ask for help.
Allowing doctors to take away these life experiences is a sad thing and an illness in society. So many gifted people have been drugged into normality and banality.
There are people who inflict mental illness on themselves. They do it to create joy and experiences by using drugs. They value the experiences and the suffer the punishment life doles out upon them because they value the positive of mental illness.
Naysayers to this would try to homogenise the human race into a placid group of things which could have been beautiful. They see the human condition as something which should be the same as a robot, that deviation from a 'happy' life is an illness and an abhorrence.
They have good intentions of course but they forget that they may be wrong. Other cultures have understood the value of emotional suffering. In places so does psychiatry. Grief, currently, is not depression. The misery - or the clinical syndrome - must be endured for 6 months at least because suffering has a purpose.
People endure physical pain to grow muscles. They go to the gym and break muscule sinew. The purpose of the pain and destruction is to create new, stronger fibre. Those naysayers to the positive of mental illness could apply their argument to physical exercise and the result would be a weak, flaccid human race where people would no longer be able to achieve the amazing feats which elite atheletes do.
There is a missing school of thought in psychiatry. The study of wellness of the mind, or psychosanology, and the interplay between wellness and illness.