Thanks, Jim, for calling this to everyone's attention. I first followed your link to the study and then went to the JVPowerTools forum. One poster had pointed out that "You specify are that the most aggressive registry cleaning option was chosen. Macecraft consistently warns users against using that option. It would be a more convincing study if the results were compared to the way real users would use the program." The developer replied: "With the PowerTools 2014 version, we have changed the way the more aggressive scan options work and changed the warnings from "do not use this" to more like "you can use this option if you know what you are doing"."
I still think the point raised by the poster is a good one. My impression is that a lot of people who use registry cleaners (including me) do NOT feel totally comfortable messing around with the Registry. Even with the change to the 2014 version, I suspect many (most?) people will still opt for a setting less strong than "aggressive," and that a study using a less aggressive study would be more useful, or at least also useful.