User blog comment:Krufix/Armor/Magic pen is anti health stacking./@comment-13287747-20130724232909/@comment-1894307-20130725083236

In the example illustrated above (Chain Vest vs 2 Giant's Belts):

When at 2.000 HP, with zero resistances, purchasing a Chain Vest and a Negatron Cloak 1.440 gold and grant you an extra 40% effective HP, ie 800 HP.

The same HP can be aquired by purchasing 2 Giant's Belts, for the cost of 2.000 gold.

That works the other way around, too. If you, as a damage dealer, purchase Penetration, instead of %dmg you will reduce the enemy's effective Health and thus kill him easier. Although what you say is true, the matter here is examined from the aspect of a damage dealer, not a tank's! :)

In the end, what he says is that you shouldn't blindfoldedly go for %dmg when facing a HP stacker. You might as well invest in Penetration as it can be more rewarding!