I'm using nginx, with a six-year old quad Xeon. Over the past 100 days, nginx has averaged a total of 4 minutes of CPU time per day.
...to serve roughly three million php requests per day, over SSL.
So, nginx isn't even responsible for .1% of my site's CPU usage.
So then comes the cache module... but literally 99% of my php hits are unique dynamic requests and cannot be cached. I could see this argument for Wordpress, or for a stagnant forum. Not an active one.