You can keep the riving knife for those cuts, it causes no issue. just can't use the blade guard. I honestly can't imagine a cut that would require the riving knife off.
Dado stack. It's a smaller diameter than the blade. You'd have to be pretty lazy to not just reinstall the riving knife when you put a normal blade back on, but I could see that happening.
I think people get complacent and if it a cut that one does fairly regularly, I could see someone saying on the 1000th cut "eh lets keep it off for this one time" and then it just becomes a thing.