A subtle distinction, Pip. Is it really sufficiently commonplace to be an issue?

I don't think it's all that subtle - people don't go out to demonstrate their contempt for the audience ("don't care"), but they do go out to give a rough-and-ready performance ("think it doesn't matter"). And yes, I do think rough-and-ready performances, & performers who admit to not having practised, are a common occurrence - in some folk clubs. Not necessarily (just to complicate matters) the ones that are doing badly.