Armor Penetration in PvE

I see how boosting a warrior's armor penetration would be helpful in PvP play, but if I'm playing purely Player vs. Environmernt, does it help? Another way of asking this is, are creatures considered to have "armor" when calculating the damage you do to them?

yes they are. Whether it be their natural fortitude..or an actual creature class (charr/mantles are prime examples) that wear armor.

It can be very useful. A well-timed armor pen attack (bow in my case) comes in real handy during the times Zealots and other white mantle use Healing Signet. It takes off a nice chunk of life. There are many other creatures that this attack is great against, and it really isn't that bad against low armored creatures either. And with my warrior attacking with good amount in Strength, I can see the increase in damage too. It may not seem that it helps always, but it doesn't hurt (except for the enemies.)