I think this is possibly true for several electronic ignition devices. The original magnetos had a safety gap screw that gave a path for the spark to jump harmlessly to earth if the plugs weren't earthed.Also if you leave the plug lead off and kick over with Ign' on it would blow the amplifier, Cheers Bill.
People may have discovered this screw when they have dismantled a magneto without removing it snd then found that the protuding screw has damaged the plastic flange on the slip ring.
It is a good idea to earth the plugs if you are turning the engine over with the plugs out.