The term "Philly Shell" is new to me; I'd never heard it until recently. What Mayweather does has its roots back in the bare knuckle days, when it made more sense to duck, slip, or parry punches than to block them directly. And, until recently, the shoulder roll was not some guru-land, mystical defensive move. That is what you were taught, as opposed to blocking punches with the left hand, which prevents you from countering with that hand.
The style that Mayweather uses is superior because you don't have to react to every punch to avoid being hit by it.