Getting negative rep isn't fun whether it's signed or not, but I agree that it's more aggravating when you don't know why you got it (which is more likely the case if you don't know who gave it to you). I'd sign mine, but I can understand why someone wouldn't. Someone might be upset by a post or think that someone is giving bad information, but they might also be afraid of retaliation. I can see this especially for relatively new members who tend to have less reputation points and are more affected by negative points. There are a number of people here who could take away quite a few points with a red chicklet. Someone should be able to give out negative rep without fear of getting it in return simply as a tit-for-tat thing. Sure, the ramification is that some people do it for fun to get others upset, but that's the price to pay for others to have that freedom.
I don't give out negative rep to anyone but spammers (who are soon to be banned anyway and aren't really affected by it). I've thought about it two or three times, but I tend to just post my thoughts instead. If I could give out/take away a percentage of my normal amount, I might consider it...maybe. I can't justify smacking someone with 150+ negative points because they upset me, though. That, and I very (very) rarely ever get upset here. It's a great group of people, IMO.