Arbalest does not penetrate all the shields and not all places. Also, thrust will be in the same cases.
Shield will be penetrated by a crossbow if:
damage > 30 + (3* resistance)
For crossbow this is fine because even if you happen to have a lower end resistance shield, chances are air friction etc make your shield strong enough. Fire rate is also low so it can't be spammed.
Besides why the hell would anyone even want to use a shield that has the chance of getting penetrated? I mean, seriously, if I just skip using the shield I can at least block the thrust without it doing damage to me instead of some random chance or constant damage taking regardless of if I block or not?
Though sure, if I played 2h or especially if I played a spear polearm build obviously it'd be awesome to make every shield user take their shield off when I start attacking them
If shields annoy you, carry an axe. They'll break pretty much any shield fairly fast anyway.
This kind of change would just either a) promote going polearm/2h build b) exclusively metal shields. It's not a problematic mechanic for crossbows, weapons with extremely low fire rate and good chance of missing someone who attempts dodging. It's an extremely retarded mechanic for melee.
I personally would rather see more variety in shield use than now, not considerably less.