League-of-legends – Exhaust priority and masteries

As I mentioned in the comments, you're looking at the wrong aspect of armor. You shouldn't be looking at the percentage it reduces, but rather the amount of damage it actually prevents as a measure of how much longer that armor lets your survive.

Let's consider an example for a champion with 0 armor. Say he has 1000 health, just 'cause that's a nice round number. Now our champion gets caught in Garen's judgement, and starts taking 100 physical damage each second (at one hit per second). Clearly, our protagonist can take ten hits before dying.

But now, imagine our champion has the 50 armor you mention, meaning the incoming physical damage is reduced. Now (still with 1000 health) it takes 15 seconds for that bastard Garen to claim our champion's life. 50 armor extended our life by 5 seconds. (100 * (2/3) = 66.6 DPS; 1000 / 66.66... = 15 hits)

So our champion respawns, wises up, and buys 50 more armor. Now with 100 armor, we're sitting at a flat 50% damage reduction. Again we run into Garen, who's still dealing 100 base damage per second. It now takes him twice as long to kill us -- 20 seconds. Once again the armor has extended our life, again by 5 seconds. (100 * .5 = 50 DPS; 1000 / 50 = 20 hits)

Back in base, we buy another 50 armor. We're now at 150 armor, and 60% damage reduction. Enter Garen -- spinspinspinspinspinspin and it takes 25 seconds before our champion dies. Again, adding 50 armor extended our life by 5 seconds. (100 * .4 = 40 DPS; 1000 / 40 = 25 hits)

So even while the amount of damage reduced per point of armor diminishes (as you observed), the amount of time armor extends your life (or effective health) remains distinctly linear.

Generally speaking? No, probably not.

Mostly because AP characters value levels over items in most cases, and in bot lane you'd be having a support sucking up XP. Overall, this would mean that the enemy's AP carry in mid is becoming stronger than the AP character in bottom lane, while the allied AD carry in mid isn't becoming notably stronger than the enemy AD carry at bot. AD carries scale with items, and not really with levels. Thus, the AD carry wouldn't really gain anything from being in mid lane.

The majority of AD carries also couldn't lane against the majority of AP carries. While AD carries would start off strong, with decent pokes, once the AP carry gets to level 6 and has a Doran's Ring or two it is pretty much game over unless they've fed. There is no way an AD carry can keep up with the raw control and damage of an AP mid. Regardless of who the mid is.

The only exceptions to the above rule would be Corki, Ezreal, and possibly Urgot. Corki actually does scale with levels, as does Ezreal to an extent. Urgot is just innately tanky with super long ranged harass. ALL of those characters would have mana issues though, and would have to rush a mana regen item to compensate for that. And Corki and Ezreal would still have trouble with high burst/high CC champions such as Cassio/Annie/Ahri/Karthus/Just About Every Popular AP Mid Right Now.

Specifically speaking? Sure!

There will always be specific situations where you could counter certain champion picks by using more elaborate strategies. This is where a lot of the original "meta breaking" strategies come from. Such as running kill lanes in bot, double AP carries, double AD carries, or even double jungle. It can work in certain situations, but it won't work on a regular basis. And will rarely if ever work in solo or duo ques.