This isn't true though. There's may be no functional difference for the receiver/merger/maintainer but there's a large difference for the submitter - this kind of approach can often be the demotivational difference between finishing and abandoning branch work.

Sure, 95% of such PRs might never go anywhere, but with the contribution pool being that small, is there a strong reason to shrink it further?

I get that maintainers have a lot on their shoulders & expecting them to be perfect receivers of outside contrib is expecting a lot, but it's also why there's so much on their shoulders.