I can't see why anyone would use that. You get the complexity of mod_perl with the bad performance of CGI? Maybe you could run your Apache::Emulator script in Apache::Registry or Apache::PerlRun just to complete the circle from mod_perl syntax to CGI-ness back to mod_perl? :)

In the Description section from the link you gave, he points out that some things, like subrequests, are impossible. So that comes back to your question of what parts of application programming are available in mod_perl that aren't in CGI. You can emulate the CGI-compatible parts in mod_perl. The only reason not to just use CGI then would be so that it could run fast if you got to run your stuff under mod_perl. But if that's the case, I think it's just as well to use Apache::Registry and not worry about it. Unless you're in a Tantalus situation where you can't use a real development environment.