Why would Varnish be required in that situation if you're already running nginx which is perfectly capable of caching a simple wordpress site in the same manner.
Sure it does, it adds ESI (which is fantastic). It also keeps backend connections open rather can creating/destroying them for each request. Varnish does restarts nicely too, we have requests that get passed on to another group of servers if the backend returns errors. It's really a very nice way of brokering http requests.
