I wish to preface this by saying I was first diagnosed with mental illness in first grade, and while my diagnoses have changed over the years (I'm now a few years shy of thirty) mental illness has always played at least a minor role in my life.

One reason why diagnoses are incorrect, or change, is because this is still very much an emerging field.  You can look at someone missing a limb and go "yeah, you're definitely missing a limb".  Not so easy to look at a brain and say "yeah, you're wired completely differently."  ...Actually not so easy to look at a brain at all, but that's another matter.  Additionally, mindset, belief, and a million other factors affect manifestation of such things, as well as the efficacy of treatment.  Placebo effect, or even possibly a reverse (not certain on that one).

Characters with difficulties are characters people enjoy.  It's no fun seeing a character that doesn't struggle.  It might not be a mental struggle, but the difficulties of those that deal with mental illness and those around them do give a greater insight into struggles in general, even as breaking a leg or handling a chronic illness might.  However, since mental illnesses tend to affect the mind more directly, I think it might be a little more direct in its effects.  *shrug*  Possibly not.  But because a lot of the affected appear normal at first, it's much easier to pretend to be normal -- and this gives insight into the troubles of a character struggling with something else, like perhaps a personal dilemma.

I would not say that mental illness automatically makes people more creative.  And I do agree that there is not really a single norm for minds.  Nor is mental illness somehow a bigger problem than so many other things in life.  Everyone has problems of one sort or another.  Self diagnosis is bogus, in my opinion, and medication is not a replacement for effort, coping skills, or control.  It is merely an aid.  I see a noticeable difference in whether or not I am able to maintain focus, and how hard I have to work at it, when I do not take my medication.  There's also a marked difference in my mood.  I should note that I come across as pretty bubbly most of the time, but that it has always felt natural to be so.  I am on a mood stabilizer, and without it I consistently feel listless and weighed down.  I was actually /not/ on it for a while a few months ago, and it was pretty awful.  People who don't produce the right amounts of insulin take medication.  If my brain doesn't produce chemicals in the right quantity to function reasonably, I don't see medication for that as much different than a diabetic's insulin.  It's just a lot harder to pinpoint and sort out.

I feel, in the end, that most rpers are escaping in one fashion or another.  I'd argue that writing and reading are similar escapes.  Video games too, actually -- anything other than dealing with the people and world around you.  There's escapes irl too, of course, but essentially anything where you can set your current circumstances aside is technically an escape.  This does not mean it's a bad thing; far from it.  The world would be a far darker place without the stories created by those who escape their life, if only briefly.  And it doesn't have to be the implied "never deals with real life" of the word "escapism".  I feel some escaping is healthy.  I mean, when you're arguing with someone, might you take a step back and go calm down?  That right there is stepping away from the current situation for a while.  And like any other hobby, rp can be beneficial, exercising useful skills and easing stress.

There's some concern about the impersonal nature of online communication, but as people are more and more connected via the internet, I wouldn't say it completely lacks the intricate social component of face-to-face relationships.  It's merely another form of interaction, and people can be just as close online as they can irl in many ways (though I'm still waiting for a way to hug long-distance.  And send food, we need to be able to send food.)  And because the internet allows a person to get in touch with a wider network of people, support communities for people in various groups that might not have a large concentration in their physical area are far easier to find online.  It can even lend a degree of anonymity, in certain cases, or facilitate sharing by people who have difficulty in person.  That component does support the impersonal argument, but again, I think it's not lacking but merely different.

It does seem to be true that a significant number of rpers, or even people on various online sites, have some sort of mental illness.  There's also a significant number with chronic physical illnesses that interfere with their irl lives.  I think that social anxiety, depression, and various other mental difficulties lend themselves better to online interaction than physical, where the "average" person might spend more hours out and about.  Before staying home meant staying away from people.  It no longer does, and socializing online can fill a portion the need for human interaction without many of the pressures of seeing people in person.  Since the ones that don't want any interaction are likely not going to be interacting online, the medium tends to naturally separate the different groups.  Also, oddly enough, it seems that a lot of the people who are the least social in person are the most social online, perhaps because we make up the difference of our interaction quota digitally.

RP, in and of itself, is not unhealthy.  Video games and reading are not unhealthy.  But anything that has an overall negative impact on a person's life should be noted and moderated accordingly.  I'm not going to give up ice cream because eating it makes me gain weight.  But I'm certainly not going to have it every day, either.  In the end, what's healthy varies from person to person.  If someone is escaping in rp to the point where they do not handle necessary aspects of life, this is unhealthy.  The same is true for someone who skips taking care of things to go to parties.  In the end, it's a matter of personal circumstances and degree.  The situation you presented, [@Buddha], "escaping at any chance", I would call unhealthy in that it actively avoids necessary activities.  But if it's "escaping as a coping mechanism so they have the spoons to handle irl difficulties better", this would be a positive.  I myself have done -- still do -- the latter.  I find rp to be a major way to unwind.  I probably do tip over a little into the unhealthy area, but if I wasn't rping I would be escaping in books, sleep, or just going out and ignoring the things that I'm avoiding.  ^.^;;  ...All of which I also do, so it's not the rp that's the issue.  It's the ignoring part.  *eyes her kitchen floor with a sigh*

I should point out that rp is not the only instance where you play as someone else.  Video games allow it as well -- as does tabletop rp, in certain circumstances, though it depends on how story-oriented your group is.  I've also done written rp in person, though it's usually far faster to type than to write.  Also easier to keep track of.  So really it comes down to what a person is seeking from rping.  In my case, it's a small part escape, but a large part creativity, mental exercise, interaction, and amusement.

The stereotype is people who rp to avoid reality and do nothing else.  The stereotype is unhealthy, but the stereotype is largely inaccurate.  Figure out what's healthy for you.  <3