The signals were developed in the late 1940's, before water ballasted gliders (besides the SGS 1-21) and back when gliders mostly sat on their noses with the tail up in the air for all to see.

The SSF strongly recommends that when the glider and towplane are radio equipped (as almost all are these days) that radio calls be made along with standard soaring signals.

I generally don't let the ground crew hook up the rope until I'm ready to launch and I definitely don't give them a thumbs up unless I'm ready to launch.