I realize it's been said, but I feel the need to stress these points:
When using any other shield against long and/or curved 2h/polearms, it tends to be very easy to find an angle with which they can just ghost through the shield, even when the shielder is facing them head on. This is a general issue that happens anyway with the huscarl, it just happens more with the smaller shields. I do it to shielders when I'm using a katana, claymore or polearm, and it's a hell of a lot easier to do against someone without a huscarl.
More importantly, arrows and throwing weapons have a higher chance of riddling you through these same angles on every other shield. I don't know about you guys, but the only reason to use 1h in battle for me (duels are a different story) is in order to use a shield that offers protection against enemy ranged fire. The only alternative is the heavy board shield, and that's way too slow for my style of combat.