You're right, in the hands of some (relatively few in my experience) really good players, the fiddle, flute, whistle, etc. can carry it off with neither rhythm nor harmony/ chordal backing. Again, in my experience, it's more likely to work for solo or duet melody than larger groups, and I was implicitly referring to session playing. I'd expect professionals to be able to do it; it's their job to be good. But the average (not power) session, with the usual mixed bag of competences and confidences, can certainly be made a better experience for both players and eavesdroppers by a good chordal rhythmic backing. I've a particular guitarist in mind, and he's certainly not me. He isn't actually a brilliant guitarist as such, but he can make almost any session gel. His only fault is the tendency to start djangoing after the fifth pint...