I've been mulling this over for a while...
It would seem that the first responses are looking at duality from a human perspective, however if we step away from that and look at it in the context of life, existance, consciousness, whatever, then wouldn't it be somthing like without one, the other wouldn't exist... a bit like braindamage351 mentioned, however with the existance of these opposing things, the opposing traits both attract and repel each other, giving birth to the constant activity of life?
Looking at it in that context, I would say that it is something deeply rooted in the structure of the universe and in essence, life is created by the dualism... or life happens because of the interaction between opposites.
(
Heh... this perspective doesn't do much for my ideas about free will however... 
)