There has been much talk lately about the upcoming armor penetration nerf, so Ghostcrawler from Blizzard decided to chime in and post some comments in the official forums.
There is rarely an ideal time to announce a nerf. In the case of e.g. the Prot paladin healing nerf, we had already decided we were going to fix it and how we were going to fix it so we announced it relatively early. The armor pen decision included more debate. I am sorry if the timing was bad for any of you personally, but we certainly can't promise that future nerfs or buffs won't necessitate gear changes.
I wouldn't focus too much on the issue of using too many of the same kind of gem. There are plenty of situations where that has been the case since BC (though it's not ideal in a perfect world). Rather the case was that classes that had been using Agi or Str or whatever were now switching almost exclusively to armor pen and being greatly rewarded for it. Gear without armor pen was being passed over. Specs that benefited a lot from armor pen were outstripping specs in the same class. As many other players have concluded, it was just too good.
That said, we don't think it will be a trash stat if we make this change. If you could beat plate-wearers in PvP before we don't think that will suddenly flip overnight. We don't think you'll feel the need to shard your items with armor pen.
We also think the change is good for both PvE and PvP. My earlier comment was to try and discourage players from thinking their PvE got nerfed for PvP reasons, though that absolutely happens sometimes when we can't avoid it. Some players have been concerned about the power of melee classes in Arenas. While this change alone isn't likely to sway their opinions, chilling out melee damage in PvP probably isn't a bad thing.
Yes, armor pen is gone in Cataclysm, at least as a rating on gear. Until then we want it to be a good stat but not one that trumps every other gear consideration. Source