We've supported pluginsync of external facts for a while (though I think we didn't back when you filed this). If there are use-cases that doesn't cover, feel free to re-open this.
Yes we have a use case.We calculate facts within manifests to deploy static files. e.g.
$bar = some_function($::fqdn)
file{'/etc/facter/facts.d/myfact.txt':
ensure => present,
content => "foo=${bar}\n"
}
Having a fact for /etc/facter/facts.d would avoid us having to hard code that path.
The obvious question is why bother to write the fact foo given we already have it's value during the compilation. We use the fact value from puppetdb for inventory calculations.