Well, the bottom line is that you need to develop a technique that will best serve the music that you want to play.
Placing your thumb behind the neck does allow free - economic - movement of the left hand fingers.
Older rock guitarist ( think Clapton) rarely use their left hand pinky when soloing, but modern players ( think Steve Vai ) are four finger players.
A complete guitarist should train themselves to adopt whatever position best serves the music that are playing.
I've no doubt that -theoretically - placing the left hand thumb behind the neck would make for faster, more fluent playing of fiddle tunes, for example.
Some great, great players ( Charlie Christian, for example) didn't use their left hand pinky when soloing but managed just fine; well, he managed a lot better than fine. He was fantastic!