A good guitar player will change people's minds, just as a cr*p player will reinforce their prejudices. I play concertina and guitar, so I'm on both sides of the fence.

Playing melody, it's wonderful to have a rhythmic groove backing you - if played well, you can let the tune wander and always know where to come back to. However, it helps if one of your rhythm players takes a definite lead on the chords and structure.

On guitar, I'll generally pick the melody, a bass line or a combination until I've worked out the chordal accompaniment from the leader (assuming it's not just a three chord trick!) then I can join in and complement the rhythm. Several people have commented that this gives the tune a "build-up" over a number of rounds.

Encourage rhythm players, but most of all encourage them to play together as accompanists and listen to the tune!