PMK is like cheating - Fine grain developers are solvent - eroding the grain for a softer effect. Sharpness is improved with a stronger grain structure. PMK enhances the grain and improves the sharpness but then fills in the gaps between grain with stain that prints as density. So you get your cake and you eat it too. Sharp grain without the grainy look. Also because it is a tanning developer - not allowing development to continue working on a highlight area after a certain point - gives you greater highlight separation and control - Kind of a win-win - Catechol also does all this with slightly less grain.
Frank