Yep, this will only add to the price. That shredded piece hanging half out of its frame is worth far more than the piece intact. Had banksy really wanted to destroy it he could have rigged up something else, but no doubt he had his eye on the art market.
Almost the whole of the market in modern art depends on branding and stunts, it might be better if more of it were self-destroying, that would save a great deal of warehouse space (most is bought merely as an investment and kept in warehouses), even if some, if it belongs to an instantly recognizable brand, is didplayed by the owners just to show thta thye can afford it.
Mhe ... it only got half way through ... the effect would have been much better if it finished and dropped the bits on the floor.
Great stunt though, it reminds me of one I read about before .... a small explosive device inside a sculpture that was set off as the 'art scene' gathered around to admire it. The sculpture was full of rotted meat and animal organs .