jaelis
Oh this is where the title goes?
I agree with you that there is a place for an armored wizard or a rogue with a shield. The problem though is that those are for the most part flavored reasons for doing so. So for instance, most wizards are going to use Mage Armor or get magical items to raise their AC because its the easiest way to do it and tends to be the most effective anyway. If one chooses to put on armor, it's not because they have no choice in the matter, it's because the player thinks "You know, it'd be kinda cool for my character's story and identity to wear armor." But the PC isn't going to really gain much of anything by doing that then if they had just gone about getting a higher AC the normal way for a wizard. And thus, forcing them to spend a feat slot to do so seems rather harsh. Nevermind the fact that something like this won't actually come into play for most PCs until 4th level at a minimum-- which means by that point they've already established how they are as a combatant and there'd be little to reason to switch over to wearing armor at that point anyway.
In truth, I personally see proficiencies as so mundane and unimportant to balancing that there's no reason to not just give them "for free" via things like Backgrounds. "If you take the Soldier background, you gain proficiency in light and medium armors and simple weapons." There... now someone who wants to do the odd character choice can do so, because their background explains how/why the can. And you can be this type of character from the beginning and also not waste a feat slot for what is essentially a flavor decision.
Well.. I don't entirely agree. With a breastplate and 14 dex, you get AC 17. To get that with mage armor, you need an 18 Dex... that is not insignificant. And of course, it costs you a spell slot, thus one less casting of shield. I think that is more than just fluff. And a shield grants another +2 AC with no real drawback if you anyway don't plan to make weapon attacks.