Antagonist’s motives: Important or not? — August 10, 2010

Antagonist’s motives: Important or not?

We all know it’s two-hundred percent important that we know and develop our protagonist’s motives.  Our readers have to come to care about the outcome, and about the main character, to buy into our story.
But do we devote the same amount of energy to our antagonists?
Do we dig beneath the surface motivations?
A movie I saw recently left me wanting a bit more from the antagonist.  I realize that it’s a movie and they’re far more limited in the amount of time they can dedicate to the antagonist.
But I wanted there to be a deeper reason for his motives than that he got jilted by a woman who chose his best friend.
I wanted the screenwriters to dig deeper into this man’s psyche.  Basically, I wanted a book of this movie.  And not a novelization of the movie.  An actual novel that told the story but went into more depth.
I wanted more about the mythology that they’d set up.  What I really wanted was to see more of the world-building, more of the internal.
Do your books leave the readers wanting more, but not in a necessarily good way?  Or do they satisfy and deliver delicious just desserts to your antagonist?  Desserts that they deserve because of their well-developed, but perhaps misguided motives?
Or do they fall flat on the stereotypical, banal, or trite motives?
Or, does it matter even?  Are the antagonist’s motives not as important in the development of our stories? If this is the case, why would this be?