I also use the 3 light. We are providing an over all condition of the property at the time of inspection. We are generalist who look for abnormal conditions that can be observed without invasive or technically exhaustive procedures. Abnormal conditions are referred to specialist who do use technically exhaustive procedures. The 3 light tester confirms the outlet is secure, has the correct polarity, has a ground pin connection (boot leg or not) and can be reasonably assumed to be functional.

I now stick with the 3 light made by Sperry after finding some other models indicate reversed polarity when 240v is across the Hot and Neutral. In any case the outlet would be written up as needing further investigation by a licensed electrician.