There is one major advantage in mod_perl
you don't have to forget: speed
. CGI itself has to be parsed each time it is called, the interpreter has to be loaded... Yes, I know that there are some fastcgi things that help there, but mod_perl is really fast.
I don't understand why it should be unfair to other OSes or platforms? Apache runs on every OS/platform one wants it to run on - at least on those which can be considered as useful OSes/platforms for running a webserver at all. So I don't get it what's unfair there. That's just a bogus reason for people who really mean IIS and not OS/platform.
Personally I even go further: I use HTML::Mason which is the greatest thing I've ever seen for website programming. I don't really know how argument handling is handled in pure mod_perl, but HTML::Mason makes it really save - one of the things cgi scripts usually are accused for. It also lets you template your site with splitting design completely and cache different components you are using for differnet times. Also session management is quite easy - just give it a try, look at http://www.masonhq.com/. Of course you can use CGI::Portable within HTML::Mason for your things, it's perl, after all.
signature(" So long\nAlfie");