Consider Comet application (aka long-polling Ajax requests). There is no CPU-load since most of the time application just waits for some event to happen and nothing is being transmitted. Something like chat or stock monitoring Web application used by thousands of users simultaneously.

Every request (one socket/one port) would generate one connection to backend (another socket/port). So each request would take two sockets or theoretical limit is approximately 32K simulteneous requests. Even using keep-alive feature on backend side does not help here since connection can be used by another request only after current one is fully served.

With FastCGI connection multiplexing we can effectively serve twice as many requests/clients.

Of course, there are applications that are limited by other resources rather that sockets/ports.

Is it really so difficult to implement?

P.S. I remember when some people were saying that keep-alive feature for FastCGI backends side would be pointless.

P.P.S. English is not my first language. Please accept my sincere apologies for making offencive statement. I did not mean to do so.