I think the rationale was that there were too many "revenge" negatives - i.e. the buyer wouldn't leave negative for the seller in case the seller left them a neg.
Maybe there should be DSRs for buyers instead [evil grin]
I'm not sure how the buyer's feedback would help anyway. When you buy from Ebay you choose the seller you feel comfortable with for the item you're buying, sellers cannot choose the buyer though. We can block people from buying from us if they have sufficient NPBs but as with my whinge above, they are getting away with it because sellers don't use the blocking tools available to them.
The only way I can see that leaving buyers a negative would help is when deciding when to start an NPB - i.e. if they have a history of it, start the NPB on day 4 regardless.