You are correct about a bad cap being uncommon. Nonetheless, having/carrying a spare cap (and rotor, plugs, points, condenser, and ignition wires) is cheap insurance (and allows no-brainer swap-out testing for bad parts).
Many years ago, when I used to work on air-cooled VWs, I looked like a diagnostic genius when I suggested that the reason a friend was stalled on the road (in the middle of the night) was a bad distributor cap. I did not tell him that it was absolutely obvious because, as he was cranking the engine, I was lucky to be looking at the engine compartment at night with no lights around. The "short line" along the almost invisible crack in the cap was very bright in the dark.
Good luck. Let us know what happens with a new cap.
