As a guitarist, I've never had any problems in sessions, but after forty years of playing accompaniments I would make one simple observation: if you're going to play improvised accompaniments in a public place, learn how the tunes are made.
Accompaniment is a craft in itself; it's not just playing a guitar, and it takes a deal of respect for what others are doing with the tune. As for the celtic melodic argument - have they not heard of the late lamented Derek Bell?