True but, OTOH, a shield already gives an additional DB (which may mean already less damage and a lower crit). Why should it allow as well a lower effect on crits?
Also, there are other ways to attain various Armor Types beyond the physical armor configuration, so even if the AT takes into consideration arm and leg greaves, a magical AT or natural AT of the same number would likely not include the greaves.
In short, both AT and DB are
generic, they apply equally to any hit, anywhere on the body (or if you prefer, to all attack rolls made, regardless of any other variables). "If foe has no helm, he is dead" is
specific, and it doesn't matter what your AT or your DB is. You either have a helmet or you don't. Realistically AT 9 should have the same problems where shots to the arms are concerned, unless you add pauldrons and vambraces. In the real world, someone whose chest and legs are armored
and that's all isn't AT 9 everywhere, they're AT 12 (hard leather, nominal "full coverage") on the chest and legs and AT 1 (unarmored) everywhere else.
I'll grant you that location specific, situational crits are a
clumsy attempt to address that... but at they least they are
an attempt to address that, and I can't say I could come up with any better solutions. I haven't seen any mechanics address damage location and differences in both coverage and attack type
well in any RPG yet.