Yes, I agree. On the P11, the harness was clipped to the loop with the small john bull ties, as the intention was the rear tail light/license plate assembly would be removed for competition. The bullet connectors would stay strapped to the loop while the assembly was stowed away during the race.
With the P11A and Ranger, the tail light assembly is not designed to be removed at a meet, so clipping the harness to the underside of the mudguard would require permanent clips and a grommet through the routing hole in the mudguard.
I used the plastic P-clips because they work well without damaging the wires, though I see no reason why you should not use the metal J-clips.
Remind me when I go to Barber this year, and I'll take a picture of the rear wiring on the P11A that is on display there (I already know there are some discrepancies on this bike, and have discussed them with Mr. Slark)