I always thought it was in case the magneto seized for some reason it would shed the teeth and not wreck anything, the same applies with the magdyno dynamo drive, strangely they all use steel mag drives, it's only ATDs that have fibre.
You should be able to machine enough away from a cam gear for it to be light enough not to be a problem.
Actually after consideration I wonder if it was so as not to load the mechanism of the ATD centre