I have experienced petite mutants arising in a single batch that was stressed in one way or another (typically underpitched, fermentation temperature issues, or similar stressor during fermentation). Once there, I am not sure how to rid the slurry of the problem mutant presence, though plating and culturing would be one way that comes to mind.
Improvement of the beer by the existence? I suppose it’s possible if that expression is something sought in the style (4VG, perhaps or clove character in a Hefeweizen)?
Others may have specific stressors that they seek in a style