> PHP is really the only major platform where an Apache module is a viable option
I think that mod_mono makes ASP.NET quite viable on Apache. So, I guess that would be one reason. Technically, mod_mono invokes mod-mono-server so I guess it means what you mean by "application server". As far as complexity goes, it is just a couple of lines in your httpd.conf (or a .conf that httpd.conf refers to).
I need to get off the Mono advocacy stage though. This is a PHP thread.