I'm pretty harsh with my negative feedback, like "No contact, no payment, not a trusted member to deal with." It's true that about half of them get pi$$ed off and neg me back, sometimes saying stuff like "this guy ripped me off" or something. It's pretty annoying. Even though I get a lot of positives, it might bury the negative but it still shows up on the "last 7 days, last month, etc" chart, as well as affects your overall lifetime percentage. I have managed to get a few illegal negatives removed, but it's hard, takes time, and expends effort that could be put to use elsewhere.
I agree that when you file a NPB alert, you should get the chance to leave feedback, but you should not be at risk for getting any in return. Filing a NPB alert means they didn't complete the transaction, end of story. If they manage to get the NPB alert reversed, which can happen if they prove they paid, etc, then they would regain the ability to leave feedback for you.
It wouldn't take eBay's programmers long to set this up, and I think it would be a good service to the community.