In all the hundreds of sessions I've done over the years, it's always been a chord chart that's been put in front of me; never, not once, have I ever seen a scale chart or any heard any mention of what scale to use when asked to play a solo over something.
The chords say it all. I really don't see any chicken/egg conundrum here. Melody, which is what music is, is not based on scales, it's based on chords ... or maybe I should say it this way: melody turns scales into chords, so if you want to learn how to play melody, concentrate on chords. They're crystallized melody.