Nobles do start with armour, they simply don't start with high quality full plate armour. Chain mail would be a common armour even for relatively rich knights over much of the medieval period, or something like plate mail. But besides adding some sort of armour scarcity to the game, I'm not sure I see much benefit either.
People can RP whatever armor they want, as it is, it doesn't change anything. So who cares if a very tiny minority of new characters RP as having full plate?
All this feature does is allow rich nobles, which are usually dukes, to become unstabable. And since dukes are hard enough to get rid of as it is, I don't see why we'd want that.
Unless full plate just gives current defense, and everyone's defenses are lowered. But then again: being wounded isn't much fun, why would we want to make people become wounded more often?
