For now, perhaps, greylisting (tempfailing) does simply make the neighbors an easier target. But once enough mail servers are utilizing the technique (which I've personally found to be very effective), then spammers will begin to send every piece of spam twice in order to get through, which WILL slow them down. A little.

Also, in the case of the criminals and the bear, it is a case of EITHER me OR them. In spam that's not so; my neighbor will be spammed whether I'm protected or not. Though I guess your point is that the less bandwidth they spend on me, the more they have for everyone else, which is likely true. I don't like the air filter analogy; greylisting doesn't relay spam to its neighbors. A better illustration might be a house in the path of a mudslide; the owner of the house puts a barrier up which deflects the mud away from his house and into his neighbors'. Or something.

Thanks for the thoughtful post.