Maybe someone is rich and just wants to mess with the network. It seems unwise to make ourselves vulnerable to that sort of thing merely because we wouldn't take advantage of it ourselves.
I think that is something to think about in byzantine threat models. Could a big and hostile player greatly out sizing bitcoin in terms of money to burn destabilize via financial exchange manipulation, or mining out the coins with vastly more CPU power, buying and deleting coins etc. It seems often that some big players prefer covert plausibly deniable or hard to prove action than something overt. Or alternatively they could find or make an legal excuse to cut exchanges off from the banking interface.
Even competitors like banks themselves if the bitcoins started to eat into profit margins maybe they could drive out the currency by buying all the liquid parts. (Bad currency drives out good?)
Adam