Buyers generally have no idea that they are hurting sellers when they give them less than a 5 star rating.
Yes, there are vindictive buyers (as there are sellers), however, the problem is eBay, not the buyers. eBay tells buyers that 3s are acceptable, and 4s are good, while at the same time, telling sellers that they'll be penalized for having anything less than 4.3 (give or take).
Punishing buyers for dinged stars is misplaced, in my opinion.