Well, the use-case that comes up quite often for me (for any commands, not just postgres) is that I add parameters I know or remember from my last look at --help and then want to check again what else to add, for that it is useful to be able to use --help as the last parameter.
Heavy +1 to fix this - it's always been a pet peeve of mine. Postgres tools are not the only ones that do this, but they are definitely an outlier.