That's terribly comical, considering it's just a graffiti piece. Banksy will find the fuzz it creates hilarious, I'm certain. Actually, the reactions it creates could be seen as the much grander art performance, compared to his previous 'art sale' [1] [1] http://www.youtube.com/watch?v=zX54DIpacNE
Only a few people want these pieces, they are worthless in a vacuum. Only when shown in the context of "These pieces are by legendary artist banksy" do they become valuable treasures, in great demand.
It distresses me the number of things on this planet that are valuable because people think they are valuable, not because they have any actual use.
I suppose their value is in manipulating others. That is a form of utility, I admit.