The Top of the World is a lounge, not a restaurant (although from 1971 to 1993 what is now California Grill was called the "Top of the World"). Is a nice lounge with a great theme and better views. "Worth" is subjective, though.
As for why Disney doesn't (or "didnt" to be precise now) have other similar venues, the answer is pretty simple. Until Grand Destino Tower opened a month ago, there weren't any other 16 story resorts. When the Riviera Resort opens in December, there will be 3. Both Grand Destino and Riviera have both a restaurant and lounge on their respective 16th floors.
Steve