I have never had a roll pin failure with a single pin. I safety wire my personal distributor rebuilds primarily because Jim Linder (gmc bubba) did that to one of mine before he stopped doing them so belts and suspenders for me.
On the other hand, my vertex magneto for 235/261 uses a single roll pin. I purchased the vertex as a rebuilt unit from vertex back in the early 80s and have run it up to 6000 rpm no issues with the pin. So I do think a single pin is no problem, that and I am with the others, rather a pin shear then have the tang get taken out on the vertex.
