I think this follow-up is needs to be read in conjunction with the earlier comment that primary is the default position.
For my commute, about 8 of the 10-miles are 60mph country roads with room for two cars to pass when I ride in secondary. I haven't had more than a handful of close passes and cars are delayed by no more than a few seconds. If I was to ride in primary, though, the fairly constant stream of vehicles in the opposite direction would mean that any cars behind me would be stuck there for a couple of miles.
If you just say that the default is primary without this proviso, I think that could be misunderstood.
As for left-hooking motorbikes, I had the opposite on Monday evening. I was on the motorbike stationary at some lights, the only vehicle in the queue and indicating left. I'd passed a cyclist a few hundred yards before the lights so I was looking out for him, which was fortunate as he was in my blind spot just as the lights changed. If I hadn't been looking for him and just moved off into the left turn, as many do, the buffoon cyclist and I would have both been chewing tarmac.