Cap'n Kobold
Hero
Actually dodging a blow is much harder and rarer than D&D would imply. Outside of simply moving back out of an opponent's range as they attack, avoiding a strike with a weapon is really difficult. Ducking and sidestepping are not a primary defensive technique in any armed combat system that I am aware of. Parrying/blocking is how attacks are generally defended against in armed combat simply because the body doesn't generally move fast enough compared to the speed of a weapon.So uh...
All the Dex in the world will let you dodge some attacks in light armor to minimize the damage you take. In heavy armor, the same amount of dodging makes no difference 'cause they'll hit metal instead of you.
Not getting Dex to AC in full plate has nothing to do with being -unable- to dodge. It's being unable to dodge more effectively than the armor already is at stopping you from taking damage.
Having said that, Dexterity in 5e is grace and balance, and both of those are important defensively. Not specifically to avoid every attack, but to avoid making a mistake or losing your footing that would make you vulnerable to an attack. This would still be of use in heavy armour simply because losing your footing was one of the few ways that you could be made vulnerable while in plate.
You could make just as much argument for Strength, granting speed of motion of your weapon and more control of your opponent's when blocking or deflecting. Outside of pure skill, both Dex and Str in their 5e incarnations are important.Couldn’t one argue high Dex would also let you parry or block more effectively besides just dodging? I imagine armor would impact those actions even less than your ability to dodge incoming attacks.