Quote Originally Posted by Greywander View Post
People say D&D isn't a fantasy world simulator, and that's both right and wrong. It's not a simulation type of game; it's not interested in using complex rules to determine in detail what happens in the fantasy world. It just tells you whether something succeeds or not, and then you come up with a justification for why it did or didn't work. However, D&D isn't a videogame-esque world. NPCs don't talk about stats or hit points. Characters aren't actually having idle chats while standing perfectly still waiting for the wizard to decide what spell to cast and finish their turn. The settings in which D&D take place are meant to be more realistic settings, where HP, turn-based combat, and so on don't exist. You'll never hear Drizzle Durden talking about which feats he took, or Elfminister talking about class levels. When someone suggests you put on some armor before a big battle, "Oh I'm not trained to fight in armor," is not a response that makes logical sense, not for a career adventurer.
Since we're emulating heroic fantasy, a logical response to that suggestion is "No thank you, I need to stay nimble", because nimbleness is a heroic trait. And indeed, under the right circumstances, nimbleness can provide better protection from blades and bows than full plate.

From that, we can inform our understanding of armor proficiency: it is a commitment to the regular practice necessary to be nimble while wearing the armor. (Note that weapon proficiencies also indicate regular practice, such that a character with the weapon proficiency can use that weapon skillfully at a moment's notice.)

This in turn suggests to me that the way to discourage dipping for proficiency is to encourage nimbleness within the character's main class. Since you're mostly seeing armor-seeking behavior in arcane casters, could you make Mage Armor easier to use? Perhaps treating it as known and prepared for everybody(without counting against total spells known/prepared), so the only cost is the spell slot to cast it, would help.