I’m not saying primary donate is better, I’m just attempting to fill in some holes in what you’ve been told. My alternate is on a necklace and I breath from it for a couple of minutes during each dive (dive day for multiple dives) to make sure it is working as expected. Not just topside before the dive. Also, because it is on a necklace, I know it is not full of sand because of it getting out of its pocket and dragging along the bottom.
Regarding alternate on a necklace so it’s easily accessible, as you said, this is a reason to put your alternate on a necklace. By default it is not donatable when on a necklace, therefore you have to donate your primary.
Again, I’m not arguing one way or the other, or trying to convince anyone, just trying to fill in some holes.
Erik