One problem. Many of these conditions are =SERIOUSLY= debilitating. Manic Depression, for example, is often accompanied by periods of uncontrollable rage. (The fictional character of Bruce Banner / The Incredible Hulk =is= the classic Manic Depressive figure, where the only changes are in the fact that Manic Depressives usually don't have to buy so many clothes, and it's not quite so predictable.)
Higher Functioning Autism (for which Asperger's Syndrome is a special case) is less-obviously debilitating. True, these people have much more focussed minds and are much more able to do -specific- tasks than your average Joe Bloggs. The price for their enhanced mental power is the partial (or even total) loss of certain mental faculties. The loss of social skills is certainly one, but that's only the most "noticable" to an outsider. There seem to be significant chunks of brain function missing, in which complex cognition is either seriously messed up, or missing altogether.
(A classic example of this is body-language. HFAs can't read it. Body-language is about as obscure as Martian, and much of the work with HFA's seems to concentrate on teaching people ways to deal with this.)
As someone who has Bipolar I (with rapid cycles, mixed states, and a teaspoon of nutmeg), and Asperger's Syndrome, I can honestly say that it offers no benefits, on the group or personal level.
Now, that does NOT mean that it is not "evolutionary". Evolution is not about "making things better", it's about "trying things out" and seeing what works. I can certainly accept the theory that all neurological "disorders" are really experiments by nature into what works.
Traditional "Autism" should =NOT= be excluded from this list, either. The people affected are more obviously "debilitated", but they are NOT simply cabbages. I suggest reading the excellent book "Somebody, Somewhere", for a more extensive description on Autism, but I'll put my thoughts here.
Autism covers a complete spectrum of conditions, ALL relating to mental focus. Those with SEVERE Autism are absolutely focussed. They will do a few things with absolute zeal, and with amazing innate ability.
There are autistic pianists who can repeat an entire piece, not-perfect, after hearing it once. THAT is a level of focus normal humans cannot achieve. The mental background noise is just too high. HOWEVER, that same "noise" allows normal humans to "multi-task" to a phenominal degree that no autistic person can ever achieve.
To put it in "computer" terms, "regular" people's brains have a pre-emtive Operating System. Any task can grab control, when necessary. This is a GREAT survival trait, and one that makes a world of difference, in dangerous situations.
Autistic people, on the other hand, have a "task-switching Operating System", where the granularity of the switching depends on the degree of autism. Higher-Functioning Autism is not unlike Windows 3.1, in that respect, with severe Autism closer to DOS, with a few routines loaded as TSR's.
Bipolar minds are much less about focus, as they are about mood. The focus is often unchanged. (eg: Most people go shopping for the week's groceries. A bipolar person, in their manic phase, might easily max out their credit cards, spend every last cent in their pockets, and then try to use an IOU to buy the rest of the mall.)
Herein lies the problem with Bipolar, though. It's NOT a trivial condition. There are 64 diagnosable forms of Bipolar disorder, each of which is significantly different from the others, in character.
It would be much more accurate to say that EVERY human being is "bipolar", but that "regular" people's minds stay within "functional" limits.
Again, I'm not arguing against the concept that bipolar could be evolutionary. Again, evolution is about experimentation, and nature is forever restless. It is entirely believable that bipolar disorder is such an experiment, but if so, it is an experiment that is doomed. The price paid by bipolar people far exceeds all the benefits, because all the benefits are illusions created by the extreme mood swings. It's a bottle of poison marked "drink me". Give in to it at your peril.
"But what about all those creative manic depressives?"
*SIGH* Their brain is a 5 litre, turbocharged engine. Mania is hitting the highs, in 6th gear, depression is switching into reverse. Ever wanted to see what happens to a gear-box, when you try a manoever like that?
What is needed is to keep the brain in about 3rd gear. Quite enough power to do everything you want (and more), without shredding the innards of the brain in the process.
This is what Lithium, et al, do. The "slowing down" that people feel is the dropping from 5th to 3rd. Yeah, SURE it feels slower. But it's also sustainable, and you don't mangle your mind by hitting reverse any more.
Being productive =ALL= the time, rather than losing half to depression and half of what's left to picking up the pieces, is the way to go.