Maybe I'm misreading this but...

If they really don't want to use up valid addresses for the point-to-point
links, why not just run the interfaces unnumbered instead?

Well, of course it is useful in a traceroute to see clearly which
interface same in through, which you can't tell with unnumbered.
I expect that most of the people who do use RFC1918 addresses on their
point to point links also resolve those addresses to names on their
internal DNS servers to provide an even nicer and easier to debug