Linoleum, kerosene, super glue, escalator and cellophane are all terms that were once trademarks and referred to specific products. As the terms were used improperly over time, they became generic and lost their value as trademarks. If trademarks are used improperly or aren't protected, the registration is canceled and-the term and symbols become part of the public domain. Most often, this transformation from trademark to general term is not done maliciously, but rather out of lack of knowledge. Suppose your new soft drink takes the consumer world by storm.
Life is good.
Your sales begin to suffer, not because folks don't like Sparkle, but because they liked Sparkle so much they wanted to try Sparkle Light, and Sparkle Light tastes like swamp water after a hard rain. Chances are you would take the makers of Sparkle Light to court. You're not after a fast buck, you're trying to protect the name under which you have created added value for a specific product. After all, your trademark and the value it represents is what you really have to sell. The folks at Wendy's don't sell hamburgers, they sell a specific quality associated with their name.