Serves little purpose in PvP considering a high armor character will hardly notice the drop and a low armor character will see a small amount (because it works in percent of total armor) so no this is not really for PvP but its great for PvE
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Essential for Rogues, warriors, and hunters; less useful for shamans and pallys due to the fact most of their abilites deal some form of magical damage and therefore negate armor anyway.
as of the new expansion pack (Cataclysm), blizzard is removing armor pen, and why? it's kind of confusing (even for people who are new to melee classes or never played them B4)