I can see why - you're already wearing a big tin can. In D&D, the call for full plate and a shield is a bit mixed:I recall reading (I've seen this in several places, I think) that as armor got heavier towards then end of the middle ages, that shields became smaller, and eventually dropped entirely because they were considered superfluous.
Point: Even if you're wearing full plate pyjamas, a dragon can still cave it in with a blow, so a shield could help.
Counterpoint: If you took the force of a dragon's blow on your shield, odds are your arm would break anyway (which under the abstractness of the HP system would be considered a hit, and do damage, I suppose). But hey, it's fantasy...
Large Shield +5 could mean it's ability to:
1) Get in the way of blows automagically.
2) Not break under the force of those blows.
3) Absorb force with respect to the wielder so that their arm doesn't break.
4) "Shove the wielder to the side" out of the way of blows with such force that it can't totally absorb them.
e.g. Although it wouldn't stop an oncoming truck, the shield +5 might absorb the blow with respect to the wielder and "shove them aside" so that the full force of the blow isn't registered with respect to the wielder. Same with a storm giant hammer coming straight down on a paladin's head; the shield gets in the way, takes some of the blow, doesn't break, and maybe "nudges" the wielder to the side. It's easier to imagine Full Plate +5 being magically slippery enough to get the wielder out of the way....although arguably this is ascribing dexterity enhancements to magical armour, it's difficult to envision how it would work otherwise, else magical armour would have stopping power, and be able to stop the truck in it's tracks, or halt 30 strength worth of hammer on a dime....and under the rules, it lacks that property, it can only stop the wearer from getting hurt.
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