Why isn't it sensible to dedicate more than one TCP/IP port to http? Although admittedly naive, isn't it  intuitive to think that server performance could somehow be increased?
Port 80 is a well-known port, which means it is well-known as the location you'll normally find HTTP servers. You can find it documented in the HTTP/1.1 RFC.

Having a default is useful precisely because you don't have to type it into your web browser with the URI.
If you run an HTTP server (or in fact any service) on a non-standard port, you force the client to remember which arbitrary 16-bit number you chose and type it in.

In addition to this unfriendliness, there is no performance benefit: a port is just one part of the (dst ip:port, src ip:port) 4-tuple which uniquely identifies a TCP connection. If two connections share a dst ip:port, that doesn't mean they share some system resource - they can reside in different threads, or different processes.

Now, if you have logically different services which both happen to use HTTP, there is no problem with running them on different ports. It just makes the URI a little uglier.
