I doubt that there are many people who have known someone with a mental illness and would claim that there is anything positive about the experience. While sometimes the mentally ill themselves claim that they have a superior insight into reality, that's just because they lack insight into the fact that they are unwell. However, what mental illness, or any other disease, does provide is a natural experiment that helps us understand the normal function of the affected organ or system. For just this reason, in medical research, one of the most common experimental tools is to deliberately cause lesions in an experimental animal and observe the resulting effects.
Yes, I'd agree with that--and besides intentionally causing lesions in animals, accidental brain injury in people can give insight into the normal function of the corresponding brain areas in uninjured people, and sometimes other types of mental illnesses can provide the same kind of insight.
By the way, on the subject of what mental illnesses tell us about the way our brains percieve time, here's a very interesting article by Oliver Sacks on the possibility that our brain strings together a series of "snapshots", in much the same way that movies work, rather than integrating sensory information in a more continous way:
This article was originally from the New York Review of Books, and you can also read some letters by other scientists written in response at http://www.nybooks.com/articles/17030