I did a little research on it. It's actually not more or less effective depending on armor. Also, there's only one PvE trinket with armor pen (DBW).
From this article,
http://www.wowhead.com/forums&topic=...or-penetration,
So, it seems that it is designed to be equally effective against both high armor and low armor targets. However, if plates didn't have high armor, ArP wouldn't be as much of a priority compared to other stats (raw AP, crit). You do need ~30% to be effective against plate. Clothies, not so much. But it still is equally beneficial against clothies.