Humans can spam too.
This may be smaller than what a completely automated computer attack could do - but I still wouldn't want 576,000 emails in my box one morning. These sort of systems may shift the financial scale of spamming systems for profit, but a system has to resist even nutty people. In general, the reason I don't find any type of POW appealing is I don't see the parrallels to the way things happen in meatspace. One of the things that has drawn me to P2P architectures is that they have so many parallels to how things happen in real social systems - and we know such systems have been in the trial and error phase a lot longer than computers. It's stupid, and it would take the fun out of swinging.