I think the guy you replied to was trolling or something, with their absolute clown take.
Civil Militia gives armor+shield proficiency which is invaluable for caster types. Right out of the gate, as an example, Gale can get 13AC from leather armor and a shield, which is the same as Mage Armor, and can then add DEX. Which means he's saving a spell slot and getting comparable AC buffs for no trouble.
Your best idea was to drop this proficiency for a skill proficiency, of all things, because...?