Hmm...
Knight: honestly, if you're using mage Knight, you're probably using Wizard Robe, because Knights are invincible in LFT anyway. Easy solution: only have the bonus on armor, not helmets, and make it less of a bonus than Wizard Robe; that way you might get Knight to ignore it--which you probably want because Knight doesn't need the help.
Samurai: get buffed significantly by this change. And that's a problem. Samurai's original design was "their class is weak, but hey: good skillset". The skillset's been nerfed, sure, but it's still good, except we made the class strong--too strong at first (got nerfed). We might want to nerf their multipliers -again- if we did this, and I'm not sure I like the further multiplier nerf though--Samurai are supposed to be a class you might use a faith-based skillset on because of the accessible Always: Faith.
Equip Armor on a mage: I don't expect this will be used still. Well, the big place it could be tempting is Summoner for Golem, but that doesn't require MA, and Summoner already has ridiculous MP; in short, people can use this already. Maybe Golem setups in different classes? Summon Magic Lancer might benefit from armor with an MP boost. In general, though, DU/MDU will probably serve a mage better as a support skill.
Honestly? Unless you give big bonuses (like...make armor that's better than Wizard Robe) it's probably not going to prop up Equip Armor over other defencive support abilities. So...you're mostly buffing Knight/Lancer/Samurai...and therefore mostly Knight/Samurai. Two classes that are near the bottom of my "might need buff" list right now.
Equip Armor could be lowered to the point that a bunch of classes might use it in Chapter 1 (if it was like, 100 JP). At 300 JP it's...deliberately set to be not an auto-pickup for Monk (at 200 JP it's silly convenient--the moment you unlock Monk you have the JP for Equip Armor; there's no decision involved. At least at 300 you might have to choose between developing your skillset faster, or spending an extra fight or two in Knight).
Hm, actually, one option would be to stick it on Lancer, so that it's not in Monk's unlock path. (On Lancer it could be like...50 JP pretty safely; might as well go higher than that, though, to make the decision a bit less automatic in Chapter 1; maybe 100 JP, so that people at least consider not buying EqA in order to save up for PA-Save or Dragon Spirit or Defence Up).
