If a player had enough gold to purchase a Huscarl shield along with their preferred weapon loadout, they would probably invest it in something more practical -like armor.
As far as punishing turtles in Warband, they pretty much punished themselves by playing that style. Not only will your shield get battered down pretty quickly -as OGL said, but a player who is turtling or passively hit-blocking has a much diminished game impact. They aren't applying proper pressure to their opponent, so thus it's also very likely they aren't keeping any fidelity in the groupfight, which means their mark can probably go make a play on one of their teammates while they themselves are playing too passively to intercept.
Turtling affected a team negatively on a both an individual and group level. It wasn't a problem for people playing against turtles, just the turtlers themselves. There was never a need to find a way/develop a mechanic to get around turtling, as the style itself is very self-defeating.
