I have somewhere around 15 or so negatives (out of about two thousand). Almost all of them are from bad buyers hitting me back. I simply don't care, anymore. I'm going to negg these idiots and scammers, and if I get burned in return, screw it. I think that potential buyers see my 99% positive rate and know that I'm not going to rip them off. I've only lost one shipment of cards, and I'm not sure if I really did. I believe the buyer may have been saying that to get a refund, but only they know the truth on that one. So generally, my negative feedback is from non paying buyers, which is kinda sad if you think about it. They screw a seller, and then they leave them negative feedback.
"there is no cash-flow incentive for them to drag this on for 10 days or so"
Except that by making it a lengthy and difficult to find/navigate process, some sellers aren't able to figure out the forms, or even locate them. Others forget by the time the 7+10 days rolls around. Sometimes a computer crash will mess up your records, or a holiday will make you miss the deadline, or you just give up, saying you're not going to keep a mental note of something for more than two weeks, just to get 20 cents back. If NPB refunds were just automatically credited (as they should be - it's our money) then eBay would lose millions of extra dollars.