We used an accoustic bass with Cobbers for years here in Australia. The model we used was called the Bindarra, made by the Maton company in the sixties and seventies and sounded great. Using the bass really ties the music together and if the volume is there it can help a session by creating a beat that everyone can work to. It was modelled on the Mexican mariachi bass but given a full fretted neck. We always had trouble when we went to America because they would argue that you didn't need to take a bass player with you but should employ an American musician. The same stupid logic occurred here in Australia a few years ago when a local film maker wanted to use John Lee Hooker in his movie and was told he had to use local musicians rather than JL's own band. (We could start a whole thread about stupidity in beaurocracy and its effects on music. Anyway, to counter this, our bass player always listed himself as a Bindarra player on work permit applications. I was the fiddler and had similar problems until I listed myself as a penny whistle player.