[AdamN]

FTP splitting the data and control ports was a smart implementation. It allowed for optimization of the ports (one focused on throughput) and meant that control could still occur even while long responses were happening on the data port.

[immibis]

I haven't tested it, but I imagine it allows you to trick an FTP server into making an HTTP request (without TLS).

It probably seemed like a good idea at the time, and there was no way to know the problems without trying it.

It also allows you to send a command to cancel an ongoing transfer.

[ksherlock]

Well... RFC 953 specifically mentions using PORT to send a file to a line printer so that seems intentional. [1]

For streaming mode STORe/RETRieve the connection closes at the EOF so you could send the request but the response would be lost.

[1]

    It is possible for the user to specify an alternate data port by
    use of the PORT command.  The user may want a file dumped on a TAC
    line printer or retrieved from a third party host.

[dark-star]

it also allowed for FXP, which was a godsend in early pirating/warez days ;-)

...or so I've heard

[sneak]

This could have been better accomplished by multiple connections to the same port.

[ajsnigrutin]

A lot harder to implement QoS on that.

[sneak]

QoS didn’t exist when FTP was invented. The protocol was designed badly.