I'd say it's still useful to know what confident, outgoing and friendly body language looks like, so you can recognize it in others and yourself. For one thing, it might help you deduce what kinds of actions make people more confident, outgoing and friendly, so you can try doing those things yourself. For another, knowing whether somebody else is confident or not can be important in deciding how to relate to them (you don't want to overwhelm an unconfident person).

It's definitely more of a sensory ('input') skill than an active ('output') skill though.