Wow. I have a completely different perspective. If I leave a neg for a NPB (and thank goodness I haven't had to do this for a LONG time!!), I fully anticipate receiving one in turn--and if I do, I appeal it with eBay and occasionally it's even removed.
Why do I risk it? I just feel that feedback should be honest--otherwise what's the point? If someone doesn't hold up their end of the bargain by paying for the item they won, I feel that it's dishonest and a disservice to other sellers NOT to warn them via honest feedback.
Yes, I know that as a seller getting a negative hurts--but any smart buyer will check to see the other side of the story via the FB you left for the sour grapes NPB person. On a similar note, it's literally impossible to maintain 100% feedback forever. You really can't please all of the people all of the time, no matter how hard you try and how honest you are. Once you get thousands of transactions under your belt, you're just inevitably going to have someone who leaves you a neg--whether it's because they're mad at YOU because THEY didn't pay, or whether it's because they just don't like you. Or your item. Or they're mean. Or some other random reason.
I guess my overall point is that I DO leave negs when I have to, and if I receive one back for just doing what I think is right, well then I will wear that neg with pride.