In protocol design, why would you ever use 2 ports?

Question

When a TCP Server does a socket accept on a port, it gets a new socket to work with that Client.
The accepting socket remains valid for that port and can accept further

Answer 1

FTP was designed at a time when the stupidity of a modern firewall was inconceivable. TCP ports are intended for this functionality; multiplexing multiple connections on a single IP. They are NOT a substitute for Access Control Lists. They are NOT intended to extend IPv4 to 48 bits addresses, either.

Any new non-IPv6 protocol will have to deal with the current mess, so it should stick to a small contiguous range of ports.