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

IIRC, the issue wasn't that FTP uses two (i.e., more than one) ports. The issue is that the control connection is initiated by the client and the data channel was initiated by the server. The largest difference between FTP and HTTP is that in HTTP the client pulls data and in FTP the server pushes it. The NAT issue is related to the server pushing data through a firewall that doesn't know to expect the connection.

FTP's usage of separate ports for control and data is rather elegant IMHO. Think about all of the headaches in HTTP surrounding the multiplexing of control and data - think Trailing headers, rules surrounding pipelined requests, connection keep-alives, and what not. Much of that is avoided by explicitly separating control and data not to mention it is possible to do interesting things like encrypt one and not the other or make the control channel have a higher QoS than the data.