Ringfree and Mercury Quickleen are highly concentrated solutions of Chevron's Techron, which is polyetheramine (PEA) and related derivatives. Techron is a well tested and highly regarded fuel additive that is a component of Top Tier gasolines. If you doubt the effectiveness of this additive, just do a search on just about any automotive forum. It cleans gum and varnish from the fuel system and prevents/removes carbon deposits on intake valves, upper cylinder areas, and ring lands. It even cleans up fuel level sending units in gas tanks.
From the technical papers that I have read, it reduces deposits by altering the structure of the particles such that the are released from metal surfaces and get blown out through the exhaust.
The need for Ringfree in both 2-stroke and 4-stroke outboards depends on the quality of the fuel you are using. Yamaha states that it is recommended for underaditized fuels. Many gasolines contain only the minimum amount of detergent additives that are necessary to pass industry standard tests. Studies have shown that in the field, the minimum level of additives often isn't good enough even in automobiles. That has lead to specific brands of gasoline being certified as Top Tier fuels. These have much higher levels of additives than brands that just meet the standards.
Outboards operate under conditions that are more prone to deposit formation than car engines. They run cooler when trolling, experience higher sustained loads, and operate at higher average rpm. Combine that with the fact that boat fuel tends to sit around for longer times which promotes oxidation, and you have an argument for using the best fuels possible.
Here is a link that explains the gasoline quality dilemma in a non-technical way:
http://www.aa1car.com/library/bad_gas2.htm
If you use only Top Tier gasolines and use your boat often and run it year round, you may not need additional additives. I have a pair of 12 year old Honda engines that have never seen a fuel additive other than StaBil. One has over 4000 hrs on it. Both still have near factory compression. But they were alway run on Chevron or Texaco gas and were used year round.
Since I now keep a boat at a marina where I can't be sure of the fuel source, I always add the maintenance dose of Ringfree. If you price Techron Concentrate in an automotive store and compare how much of this less concentrated product that you need to add per gallon of gas vs Ringfree, the Yamaha product is not that bad a deal.