User blog comment:ClariS/Escalating Return: Damage Reduction/@comment-3450702-20120816112708

What you're saying is that when you have lots of Armor/MR, it is more valuable to get % reduction than more resistances. While this is completely true, and it's cool you mentioned it, this is generally the case- you are promoted to buy what you have less of and discouraged from what you have a lot of. If you made the same table with, for example, an added 500 hp (1k or 1.5k in separate columns), you'd see that the more you have of resistances, the more each point of health counts.

What I'm trying to say is, your point is fine, but doesn't apply just to these two. In fact, for almost everything in LoL, if you have tons of one stat it's worse than spreading your money in more stats. This applies to mainly two things: tankiness (you want a mix of hp and resistances, with some % reductions and all that based on the champion) and physical DPS (a mix of AS up to 2.5, crit up to 100%, AD, crit damage, and ArPen), with slightly less obvious relations for magic DPS (a mix of CDR, AP, MPen and enough mana to sustain it) and on-hit (AS and on-hit effects)

For physical DPS: With an attack speed of 1.0, and 0 crit chance, no crit damage, and a base AD of 100 a bloodthirster adds 100 to your DPS. Now, imagine we already have a few items affecting these stats, and we have 2.5 AS, 100% crit and 50% crit damage, and an additional 100 damage (~200Damage) (2PDs+Trinity+IE for example). Now, each of your attacks crits for 250% damage, and you do 2.5 every second. 200*2.5*2.5=1050 DPS. Now imagine adding a bloodthirster for an additional 100 damage-this adds 525 DPS. A lone bloodthirster would only add 100, a 6th item thirster could add 525.

Anyway, tl;dr: stack everything and you'll get worse results than getting a bit of everything. And the more items you have the better your next item will be.