because the install that i went with was compatible with WHM and it was a 15 minute chore to switch over
to go fully nginx and ditch httpd woulda been a bigger task.
it's possible, via PHP-FPM, but the ripple effect to things like awstats, webalizer and WHM in general wasn't worth it to me
i had a box with 10 domains on it and 3 ubbthreads sites on Eastern Time zones, so i could experiment in their 'night'
when it worked, i was happy.
i also went with the latest mod_pagespeed for apache and that helped a lot too..
if you have a free server to play with, i'd suggest going for the whole ball of wax, if you want
http://bethesignal.org/blog/2009/04/06/replacing-apache-with-nginx-for-static-file-serving/ <-- that is essentially what i did
another overriding reason was that i use mod_rewrite rules a _lot_ and nginx ain't up to the task ( yet ) like apache is.
essentially enough reasons for me to go with the static file proxy model
my current preference is: nginx ( static files ), apache + eAccelerator for php (fast cgi, so i don't run into file ownership probs with php as 'nobody'), mod_pagespeed, mod_rewrite
tested on 3 WHM/cPanel servers: 1 server is low traffic, 2 servers are high traffic
end result: significant performance gains
