No, feedback isn't useless but eBay has rules about what can or can't be said. Why yours was removed we can't suggest unless you let us know exactly what you said.
IMO eBay made a big mistake when it removed the limit on the number of words allowed. Now irate buyers ramble on forever! The more they say the more likely they are to say something that isn't allowed. Then the seller can simply report the feedback for removal. It happens all the time.
The best advice is to keep comments short, factual and relevant, when they won't be removed. In fact, it's near impossible for sellers to get eBay to remove feedback unless the buyer gives them a helping hand!
