Feedback is Useless : eBay Guides

If i buy an item, and it turns out to be a crappy product, i feel i should warn other's.  I leave a negative feedback and it pisses off the seller so they leave me a negative feedback. Why should i get the negative feedback? I pay on time, and leave honest feedback. Is that not the point of feedback? to keep sellers competative? sell a good product and you wont get Negative feedback.