I don't doubt your knowledge on this at all.
But do not all of the black wires already connect together and ground themselves to the engine? The only reason I can see for a frame to engine ground is if you chose to use the frame as a ground for one of the components (like how pvl coils ground to the frame.) I can see no reason in a perfectly functioning ignition system why the voltage would choose a path other than back through the ground back to the engine that is already in the wiring harness....but electricity does funny shit I guess.
If there is a short, bare wire or malfunction of sorts, I can see where it could be a safety issue with getting shocked....but this can happen even with the frame ground. The way I hook up the tether (one lead to each side of the coil) this has happened before. Usually just swap the leads on the tether and this goes away.
My bikes have 1000s of passes and/or dyno pulls with no frame to engine ground....or any part of the ignition grounded to the frame for that matter.
I didn't come up with this idea on my own, in drag racing it's "monkey see, monkey do!" I saw how someone had their ignition wired with a very simple method, and that guy definitely had his shit together. We discussed what was going on with the wiring and he said basically what I'm saying.
I appreciate the technical info on why the ground could be needed...and the rationale for it.
If (very rarely) we have to switch electrical components to rule out issues, it only takes a few seconds the way all of mine are wired.