I don't think that it is an ideal way to measure ethanol content. Most OE's are going this route as it is cheaper and easier to do. The issue is, the ECU is looking for a lean condition to assume you are running higher ethanol contents. In our case, as ethanol content increases we want to enrich the fuel mix and also increase boost and timing in many cases. The problem is you could have a single injector clog or start to fail. The ECU will take this as higher ethanol content even though it may not be. With one lean cylinder adding boost and timing in would be really bad.