You called his statement ridiculous, but you also kinda validated what he was trying to say.
I don't see how I was validating the statement "the best reason to donate the primary is that it keeps (or at least should keep) people from buying really crappy backup regs..." Maybe all the other explanation was confusing, but I thought I clearly explained what I thought the best reasons for donating your primary were: 1. It's not unknown for a panicked diver in an OOA situation to go for his/her buddy's primary so you might as well practice it that way. 2. Avoid task loading an already nervous diver.
I don't think the best reason is because it encourages people to spend more money on a backup. What many divers I've been around need...and I've been around a few...is something that will safely get them back to the surface. In an OOA emergency you're calling the dive and heading up. The backup doesn't have to breath as easy as the expensive primary. It doesn't need a bunch of bells and whistles. It just needs to work and most octos and lower end regs will do that if properly maintained.
If you think people should spend something then encourage them to spend more time practicing the skills to resolve an OOA situation safely, promote the concept of gear maintenance and, of course, persuade them to practice good gas management skills (Always a good way to avoid being OOA, right?).
It's good that you have a calm demeanor. You'll probably make a great DM. Unfortunately, there are many people who don't and there are plenty of us who have seen someone grab a reg out of someone else's mouth. It happens. It's the worst case scenario and it's rare, but don't you want to plan for the worst case scenario?I would like to point out that the two times I ran out of air, I didn't grab a regulator out of anyone's mouth...