I'm sceptical about the use of acoustic basses in tune sessions. I think it all depends where the player is coming from. Folk guitarists who have switched over to the bass tend not to be familiar or steeped enough in (for instance) Irish traditional music and it's relation to the dance tradition to have the sort of intuitive understanding coupled with instrumental dexterity that would enable them to enhance rather than impede the music. I played for a while in a session in Liverpool where there was an electric bass player and while a nice bloke he clearly didn't have a clue about the music. Plus if the bass player starts plugging in you're going to get other players doing the same. I tried playing electric guitar as a melody instrument a few times in sessions in London and while I (sort of) got away with it I was never very comfortable with it as it just didn't sound or feel right to me. Nowadays I alternate between playing rhythm with a strong bass line and picking out tunes on the acustic guitar