[aetherson]

I guess the frustrating difference is:

Suppose that the natural situation, with no SEO at all, is that you have the #1 site on some keyword, and your competitor has a lower ranking.

With "positive" SEO, okay, your competitor might have paid a lot of money and bumped you to the #2 site, and that might be frustrating -- but you're still #2!

With "negative" SEO, your competitor pays a lot of money and now you're #100. And that sucks more for you.

Probably not important in the really big keywords where all the top sites have always paid big money for SEO -- then, sure, it's all just relative. But in the relative backwaters where lots of sites aren't engaging in SEO at all, having negative techniques work is a lot more frustrating to the people who just don't want to play the game.