I had a real-time talk with a friend about the issue, and discovered in a few minutes what it took us a couple pages to really bring out here: We don't all share the same assumptions about the role of medium armor in the first place, so we can't really relate to other viewpoints, because they are aimed at different goals.
So I'll clarify how I see the role of medium armor, and armor in general. As a general rule, heavier armors should provide more benefit. The more armor proficiencies your class gets, the better AC you should be expected to have. All three armor categories should address equal categories of users. (Ie, "Strength focused" and "Dexterity focused" are equal categories, but "Requires both Strength and Dexterity" is not equal to the others, as it is a category that requires greater investment to get into. If the armor type that focused on that category were the clearly superior armor type overall, that would be probably be okay, but since it isn't, we default to the assumption of addressing equal categories of users.) I visualize Dexterity focused combatants differently than Strength focused combatants. The traditional warrior is a Strength focused combatant, who can wield a longsword well, while Dexterity focused combatants thing slippery folks who I do not see traipsing around in scale armor or half-plate. Scale and half-plate to me look like something a Strength focused combatant should be wearing. I also do not believe any armor category balance that is level discontinuous--ie, it's great at low levels and awful at high levels--is a proper armor balance.
To summarize my premises:
The roles of armor categories are to provide good primary options for equal categories of characters over the course of their adventuring careers, in a manner that more or less supports D&D history and lore.
So, coming from those premises (and about 4 more I'm probably not even thinking to bring up) my concerns might make more sense.
Without feats, a light armor wearer can top out with an AC 17 and no stealth penalty, while a Dex (or Str and Dex) medium armor wearer can top out at AC 17 with stealth penalty, or AC 16 without, and a heavy armor wearer tops out at AC 18. That is just wrong to me. (My system isn't even going to fix that, BTW, it's just going to mitigate other issues. This is a remaining issue that isn't a big enough deal for me to worry about fixing.)
Requiring both Strength and Dexterity from a character for one armor type, while other armor types only require one, and not making having the dual attribute focus provide clearly superior armor fails for me.
(This was a good thought on how the system could have addressed that:
That creates the same problem that you dislike, punishing a high STR build with a 'need' for good DEX, as well. I think it'd be nice if medium armor were part of a cluster of mechanics that made OK-good STR & DEX builds more viable, rather than 'punishing' high-STR builds. Not sure what that'd be, exactly...
Edit: Ok, I thought of one. It's a little fiddly...
When you attack with a one-handed weapon (other than the rapier), a versatile weapon used two-handed, or a ranged weapon (other than a crossbow), you can combine your STR and DEX modifier to attack and damage, up to a total of +4.
I think that's a great idea, although I don't want to house rule that far. I'd have loved it if the game were designed that way.)
A valor bard or ranger wearing medium armors ought to be a Strength focused character for me, but they are punished for doing so with the current rules. (Unless they also have that 14 Dex, which I addressed above as setting up an unequal armor category requirement.)
If medium armor is better than light at lower levels, and then equal at higher levels, that's fine. Medium armor
should be better than light. It probably shouldn't be better than heavy at any level. Note that my goal isn't actually to make medium armor better than light though. It is only to make it a good option for characters that make sense wearing it.
With bounded accuracy, mucking about with AC is always an issue for monsters and balance. and if fully a third of potentially affected Monsters see a rise, that could be a problem. Doubly so if they are mainly low level grunts who need the armour boost.
...
Basically, I'd increase CR of those monsters accordingly, or ignore the medium armour buff for them.
Yes, I have to be careful here. In general, an AC increase of 1 doesn't raise CR, but an AC increase of 2 might, and an AC increase of 3 is quite likely to. Ogres are the only +3, and I'd just change them to "studded leather" so they keep the MM AC. Duergar and Half-ogre are the +2s. That gives a +1 to four others, with 15 unchanged. What I'd actually do is just ignore everything but the Duergar and Half-ogre, and I'm not sure what I'd do with those actually, but I don't think those two monsters are going to hold me back.
Therefore, the only people who lose out are non-traditional melee characters - STR bards and Rangers who want to drop DEX below 14. And I would ask the questions, is this a) a major problem, and b) warrant a rule change. In both cases, I would answer: Probably not.
My answer is "Yes", which is a good indication of some of the differing premises on the topic. Those are classic class concepts for me.
Unless you plan on taking Dex above 16 as a second stat (to the detriment of other social stats or CON), then having Medium Armour in the majority of cases is better than not having it. Therefore it serves it's purpose for existing - as a gap blocker for certain classes.
I don't see that as a good role for an armor category. It's a different role than the other ones, which are to provide good primary options for equal categories.
[quote I can see how, if you are making the choice to pump DEX for combat optimal reasons, it might nark a bit. But if you're only concern is combat optimisation (oh I'm sorry - I meant "Progressing your character normally".......) are you really taking sub-optimal choices such as a melee STR Ranger anyway???[/QUOTE]
Sub-optimal is a matter of degrees. I'm currently playing a character that is all kinds of sub-optimal. High Charisma for no class benefits, that sort of thing. So I don't make all my characters optimized. I almost never take any stat below 10, and always have one or more 12+s purely for role-playing. I just think there are points where the rules go too far in penalizing choices. And making a longsword wielding ranger invest in Dexterity or lose 2 AC over the finesse ranger is pushing into that category.
I don't see a huge problem with it. Adding STR to AC may seem strange. Adding the /lower/ of STR or DEX to AC could 'realistically' represent high enough STR letting you move more or less unencumbered by the armor, which lets you take advantage of DEX if you've got it....
Yeah, it takes a bit of interpretation to fit it in without doing something unfortunate like that.
I have a better solution than the Strength or Dexterity mod after some discussion:
If you wear medium armor, you add your Dexterity or Constitution modifier (your choice), to a maximum of +2, to the base number from your armor type to determine your Armor Class.
This came as a suggestion after my friend indicated some very different assumptions--including that medium armor is good for characters with neither Strength or Dexterity focus (like certain casters). I disagree that that should be an armor design goal (see mine above), but switching to Constitution fixes that issue. Since medium armor works just fine if you drop the ability mod entirely and just raise all the ACs by 2, this works fine for PCs, most of whom have a 14+ Con. So since Constitution is already a secondary or tertiary attribute for pretty much every D&D character, all this requires is that you don't stick with a Dex 12 on a character you want to get the most out of medium armor with.
I even compared it against the MM statblocks, and it effects precisely the same numbers of monsters in precisely the same way. (Two monsters switch around so that a +1 becomes a Same on one and a Same becomes a +1 on another, but the final results are the same.)
In either case, the fluff is basically that you are using your physical fitness to better withstand blows to your armor. Heavy armor doesn't care, because it basically covers you so well that it just deflects off the attacks. The simulation-level is a bit strained by 3e standards, but it's right in line with 5e standards.
I also really like the idea of limiting light armor Dexterity mod to +4, but I'm not sure that rule is needed. I'd have liked it if the game came that way, though.