Medium Armour isn't so much the "I don't really care" of armor so much as it is the "I'm salty because I can't have my cake and eat it too" of armor
To restate some eloquent points from upstream...
Most characters who aren't specifically STR built are probably (if they're sensible) going to be DEX attackers, even without a maxed DEX (because they are smart cookies and are dumping STR). However, lots of those classes aren't in a position to max DEX because they have, you know, actual class abilities to worry about first. That's a lot of character types right there. So what does a PC with only moderate DEX and no HA proficiency wear into battle? Medium Armor, working as designed since 2014.
To restate some eloquent points from upstream...
Most characters who aren't specifically STR built are probably (if they're sensible) going to be DEX attackers, even without a maxed DEX (because they are smart cookies and are dumping STR). However, lots of those classes aren't in a position to max DEX because they have, you know, actual class abilities to worry about first. That's a lot of character types right there. So what does a PC with only moderate DEX and no HA proficiency wear into battle? Medium Armor, working as designed since 2014.
