I have joined this thread rather late I am afraid, so forgive any repitition. I run workshops at festivals targeted at new and aspiring singers and one of the fundamental reasons for people attending them is because they themselves seek to 'improve' some element of their 'performance'. The critical thing to 'tease out' is what they wish to improve?
If it is a straight forward case of trying to impress then forget it, but if people are trying to make a genuine attempt to improve their interpretation of the song so that do beter justice then great. Of course, to do this effectively then skills can be learnt and developed through practice and a 'spin off' might be better delivery BUT do it for the songs sake not for your ego.
By the way, myself and Rob Harbron are running a day workshop on song and accompaniment in October at the Lewes Folk Club ..... sign up now!