You forgot to add the Primary pisses off many motorists - 99% of whom are likely to misinterpret your intentions. Pissed off motorists are a risk.
- When you're in secondary, vehicles are more likely to try to 'stay' in your lane when they pass, so you're more vulnerable to close passes. In primary, they've got to cross the line anyway and so they're more likely to give you room.
- In primary, you'll stop a proportion of vehicles from trying to pass you at all. Personal safety when you're on your bike is as much about controlling the space around you as anything else.
- As has been said, if a driver is going to pass you close he can and will often do this wherever you are in your lane.
- If you're in primary, you've got a safety zone when he does this.
- In primary, you're easier to see.
- In primary, (in my experience) you generally get more respect as you are seen to know what you're doing).
I am coming to terms, later in life, in riding primary. I do not do it as much as some here or my instructor suggested. That is because AFAIK the primary argument is theoretical - albeit well reasoned. However it is a strategy that is difficult to test scientifically. It is a matter of faith.
So I use primary when there the advantage is immediate, clear and the secondary position is particularly dangerous. In other words I do not do it automatically and in a 50/50 situation would go secondary.
I am as unhappy at this situation as primary evangelists. The key is educating motorists (not to mention planners and the police) of seeing primary riding as responsible safety choice. I despair every time I get a VED renewal from Swansea that it is selling me number plates or something else I don't need - NOT a leaflet updating me on changes in legislation, driving & riding practices etc. That is why as both a rider and a motorists I am trained to ride secondary (in 1950s) and drive with the lane discipline of the 1960s.
Driving & riding has moved on. Well not for too many of us ... the dissonance is a real safety issue which nobody addresses when a new bit of road legislation is easier if ineffective.