Well, initially I thought the same way as you do, but now I realise just why it's such good advice. Narrow country lanes or smaller city roads with parked cars either side, for example - ride at the side, and an oncoming or passing driver will sometimes pass you at speed, forcing you into the hedge or parked cars. Ride in the middle, and you can bring them to a near stop, before passing each other carefully and safely. If the driver plays chicken, you can still dodge left at the right moment and you'll have slowed them to a somewhat safer speed, moved them over, and got reduced risk to yourself as a result. Most drivers would of course pass nicely regardless, and would slow anyway regardless of your position.
OTOH there are plenty of situations where I'd ride in secondary for extended distances. That doesn't change the fact that the default position is primary.