I have seen your harmonic tach once before in a post. Never seen one in use though . Seeing one is a good thing. Next time someone is at an auction and going through one of those great boxes of stuff will know they just didn't stumble on an old Afro Pick. One tach I saw demonstrated, scared the peewaddle out of me, was one that looked like a starter and you held it onto the spinner and read the rpms. I put that right up there with the old clear plastic props.
As John mentioned, tachs have there place but most the time they just sit in the flight box. I tune a lot of different engines and some of them require a tach for one reason or another. The needles on one of my Brison engines face the prop and have about a half inch of room before you loose fingers. I tach the engine then shut it off, make an adjustment then fire it up again and take a reading. I do this when I change props. I have a couple of YS engines you adjust with the regulator, you can't hear an engine change but you can see it on the tach. They do have there place but don't get used all that often.