I am not understand the reasoning/math behind this.
That would be part of the article, though it might be too dry for a paper called Fight On!
1HD monster has a 15% chance to hit a guy in full plate and 10% chance to hit a guy in full plate+shield.
1HD monster has a 55% chance to hit a guy in no armor and 50% chance to hit a guy with a shield.
Full plate guy reduces expected hits by 33%! Unarmored guy reduces expected hits by 9%. The shield is more than twice as effective for the full plate wearer. By instead having a shield throw (say shield blocks 1 in 6 attacks that would otherwise hit AC) the less armor you have the more hits the shield will prevent (since you roll for it more often) giving a greater benefit to a less armored character.
This also means shields are more effective against stronger monsters than against weaker monsters as the armor is relatively weaker against their high HD attack. So against a horde of weak monsters a full plate and two handed sword might do better but against a strong monster a shield will serve you better. My rule also includes shields being inherently weaker against larger monsters (who just smash through) so shields will be the choice vs. strong normal sized creatures like high level fighters, vampires etc. while two-handing will make more sense against ogres, giants, dragons etc.
In the end, what bothers me is both the ahistorical stacking of heavy armor and shield, but also the fact that stacking heavy armor, shield, Dexterity, rings of protection etc. becomes more effective the bigger your stack is, instead of spreading the protection out among the group. The ring of protection is WASTED on the unarmored mage but might cut the expected hits a fighter recieves by a quarter, a third or in half depending on the HD of your opponent.