...My husband is of Italian descent -- half Calabrese and half Siciliano. His mother says that in Calabria, from town to little town across the mountains, sometimes you couldn't even understand the dialect they spoke. Same thing in Sicily. Isolation, both cultural and geographical, and profoundly impact the language spoken.
Based on my wife's experience, speaking a Calabrese dialect when you're shopping in Milan is a great way to get thrown out of the store<grin>.