I like to think of it this way.
When you buy a car, you base your opinion of the vehicle soely on the merits of the car itself, completly ignoring the waste material generated to produce the car, or the mental and emotional state of the engineer who designed and built the car, and so it goes for art. We should only base our opinions of the peice on the merits of the peice alone, the waste generated, or the failed attempts should bare no relevance to the final, completed peice.
Sure, it's often interesting to know the background behind the peice, to understand a little of what the artist was going through while creating the piece, and yet it's understandable why an artist would like to keep this from us. Lets not be nieave here, I doubt very much that any artist hit upon a masterpiece first time round.
Then again I could be wrong.