tl;dr: OP is right about "sweet spots", but it's worth bearing in mind that Armor does not give absolutely diminishing returns, but merely diminishing returns relative to Health.
Also, my Capitalization is completely out of Control here, because I'm not sure which words merit Capital Letters in this context. I'm too lazy to fix it.
Armor does not give diminishing returns in an absolute sense. Each point of Armor increases your effective HP by 1%, whether you're going from 5 Armor to 6 Armor or from 300 Armor to 301 Armor. This took me a while to understand, because the damage reduction formula is obviously non-linear.
Explanation: Effective Health
"Effective Health" means "incoming damage you can take before you die". This is the most practical definition of durability in this context.
Effective Health (vs. physical damage) = (Nominal Health) * (100 + Armor)/(100)
For example, let's say you have 1000 Nominal Health. (The Health meter says "1000".)
- At 0 Armor, you have 1000 Effective Health versus Physical damage.
- At 5 Armor, you have 1050. (+5%)
- At 100 Armor, you have 2000 (+100%)
- At 200 Armor, you have 3000 (+200%)
So the returns aren't diminishing in an absolute sense: every point of armor gives +1% Effective Health.
However, every point of Armor (or MR) you buy makes Health more effective. This means that as you buy more and more Armor, Health starts looking more and more attractive. This means that there are "sweet spots".
To explain further: each point of Nominal Health gives an amount of Effective Health proportional to your resistances. If you have 200 Armor, for instance, then you're basically getting three Ruby Crystals of Health for the price of one, at least with respect to Physical damage.
So really, it's less that Armor gives diminishing returns, and more that Health gives increasing returns (relative to Armor).
Why This Matters
This might sound like an academic distinction, but it's important when considering (e.g.) Armor Pen. I used to think of cutting someone down from 20 Armor to 10 Armor as being more important than dropping them from 200 to 190, because of the larger change in percentage reduction. However, the change to effective HP is the same. In other words, Sorc Shoes are just as valuable whether your target has 30 MR or 500 MR. (Well, I mean, except in the sense that I don't know why you're throwing Magic damage at someone with 500 MR; surely there must be a better target.)
Although I haven't fully error-checked this intuition, it occurs to me that this also means that cutting a high-Health Champion's resistances down is more powerful than cutting a low-Health Champion's resistances, because you're reducing a Health multiplier. In other words, let's say you're a shredding Champion and you're trying to help your team destroy the enemy Champions. Your allies will deal 5,000 points of damage. (Let's say it's all Physical.) You'll contribute more to the cause of killing the enemy team if you shred 20 points of Armor off of the Champion with 5,000 Nominal Health (result: enemy team's total Effective Health drops by 1,000) than if you were to shred 20 points of Armor off the Champion with 1,000 Nominal Health (result: enemy team's total Effective Health drops by 200). That said, there are many tactical reasons why you might not want to target the 5,000 Health Champion first.
, gives you both
+
plus her w and watching to figure out if you can actually see the damage turrets deal to you...
