Before I start on this theory, let me make a few definitions of terms I will be using. For this theory, a soul is the part that the Wheel weaves out into a new body and the spirit is the memories/personality developed during that incarnation. I also feel that Shai’tan is a very powerful spirit, with no body or soul.
I believe that when Lews Therin and his Hundred Companions sealed Shai’tan, they had to construct something that would hold his spirit captive. They used the seals, but Shai’tan’s counterstroke caused their spirits to be bound to saidin. This drove them mad, and since their spirits are still bound, causes all other users of saidin to come into contact with the spirits of Lews Therin and the Hundred Companions. I recall Verin saying that most men going mad complained of "voices" in their heads. These are the spirits that are still bound to saidin.
Only by destroying the seals will the spirits be released, and saidin cleansed.
wotmania says: I do not know if it is right or not, but this is a great theory. Why is Lews Therin always screaming for the seals to be broken? Because he wants to be set free... Very interesting idea. Also, I think it was Cadsuane who asked Rand if he was hearing voices yet, and this theory would explain that as well. Nice idea!
...but doesn't the fact that the taint has already been cleansed, and not in this fashion kind of pull apart this theory. How about this...
Shai'tan is the antithesis of the creator, perhaps he is some sort of personification of the "sloughed off" junk and bad stuff that was cleaned up prior to the creation of the Wheel (Creator cleans off the 'workbench'... oops, there's Shai'tan on the floor!
). Anyway, this junk or basic underlying evil is now the Dark One, and he has access to the stuff that makes him him... evil (a la the True Power/True Source, don't remember proper name).
The Wheel is 'powered by' the interaction between the two sides of the OP, right? (right) Channelers tap into sides of the power. The DO dumped some of his essence (evil and nastiness, etc.) and that caused the taint... touching the essence of pure evil and all the refuse of the universe...
I know, that's pretty much what Jordan said, but nonetheless, I think it's about the best way for me to explain how I see this.
First of all, Lews Therin is still in Rand's head after the cleansing, second of all, why would people spirits cause them to rot alive (i recall that being one of the effects of the taint)?
Cloned 
Master thief 
channie 
Warder
Its good to be bonded to a green 
Uhmm... I could be wrong...