Psimet2001 

They work great on old drivetrains. Specifically 8/9 speed and below. The tool itself is a bit too wide to work well on modern stuff (although I still use it as a point of reference). I have without a doubt had my Rohloff say a cassette was good only to have the drivetrain issues be eliminated by replacing the cassette.

This is similar to chain wear now. Elongation isn't the only measure of chain performance. the chainlines are so short now and angles so great that lateral stiffness is playing a larger role. We have run into chains that are worn enough laterally that they no longer perform well but measure well under what we would consider to be replacement level for elongation.