[jimjag]

"In theory if you have enough RAM you can go faster than nginx's on-disk... in practice the sweet spot for the gain is small and they're both on par"

Not in theory, but in reality. Even if you don't, varnish's file on-disk implementation is just as good as nginx's if not better.

The idea behind a cache is to increase performance. If that is the priority then you would pick a design and implementation geared towards that problem set. Sure you can say "Well, nginx or httpd is 'good enough'" and that would be right. But when nginx or httpd aren't good enough, you need varnish. And even when you don't need varnish, it is often a better architectural design to break out specific functionality (best of breed).