Certainly the different degrees of madness in "Hamlet" also contain varying and corresponding levels of insight.
Ophelia, the most clearly and completely insane of the dramatis personae, has a terrible understanding of the secrets and ultimate fates of the other characters; she can sum up their peril easily, with a single flower given them!
Our Prince of Denmark, however, has far more fickle bouts of insanity; he is "but mad north, northwest." His monologues show us that he is more than commonly insightful; he struggles alongside a select paramount of humanity with his questions of life and death and purpose and meaning. Half-sane, however, he remains grounded by confusion, remorse, conscience, and as a result, flounders in indecision. He can never achieve such complete abandon as Ophelia's madness allows; he can never drown in it, as she dis, figuratively and literally.
In "Hamlet," madness affords a certain clarity; becomes like a second sanity--often sees a lot more than the conventional-sanity of the other characters. Consider the sheer ridiculousness of Polonius's "sage advice;" think of ordinary Osric, obtuse and comic. Their sanity is like a madness, blind to all the things which really matter, such as their ultimate fate in the play! Those mad, characters, however, can catch a glimpse of their significance, perhaps even foresee the fates of others.
I've been warned against making "Hamlet" into a play about the duality of madness--any thoughts?