I don't add it every time (it is like $30/qt) but the Yamaha Ring Free is said to prevent/reduce carbon buildup inside the engine and "promote complete combustion" whatever that means. I have had no carbon-related problems in my outboards during many hours of use over the years, and also know others with similar good experiences. I run like a tankful (95 gal) of Ring Free treated gas at the maintenance ratio twice a year... so far so good.
Found the Yamalube 2M last week at D&R in Green Brook for $30/gal or the 2.5gal container for $66. Prices sure have increased (like everything...)