In the art world, Banksy is seen largely as trite and is known to attract new and dumb money. If you've seen his film "Exit Through The Gift Shop", Mr. Brainwash, the aspiring artist he portrays, is quite similar to how anyone who knows or cares a little about art sees Banksy himself.

It isn't just "big money" saying this - ask any cool (poor) artists or curators if they like Banksy and you'll be laughed out of the room. He's the Nickelback of contemporary art. It's not because of his message - lots of other artists make biting, hilarious attacks on global capitalism and consumer culture, if that's what you're into.

Also, as others in this thread have pointed out, practices like this shredding don't actually devalue the piece, and may in fact increase it's value. The shredding is an empty gesture signifying the rejection of a culture that the piece is very much a part of. If a piece is just an investment vehicle, what effect does humiliating the owner of the piece have on the piece's function as an investment vehicle, if it does not lower the value? They (allegedly) own the art only for it's value, so how does this strike back at them?

If you want to look getting money out of art, you might start by looking at the early performance art tradition and what it achieved in terms of decommodification. Pay special attention to the amount of lying and inconsistent storytelling. Ask yourself whether "Exit Through The Gift Shop" (grossed 3MM in USA box office and set Banksy on the track to these $1mm sales) achieves any of the same decommodification.