I'll be honest with you. I'm not sure why this is turning into such a big deal is to everyone. I'm not saying I don't understand the mechanics of the change. And I do see the impact to us Hunters, especially endgame raiding Marksman Hunters. But really, let's take a step back and look at the reasoning behind the change.
Ghostcrawler said that they implemented this change after noticing everyone was picking up Armor Pen at the expense of their core stats. Really? And somehow we're surprised that when a secondary stat becomes more important than a class' primary stat and it forced a nerf? It seems more of a correction in my opinion.
If you're a raiding Marks or Survival Hunter you should be focusing on picking up as much Agility (Attack Power if you're a Beast Master) as possible. Hence why in Cataclysm we're seeing things like Armor Penetration removed from the itemization.
Think about it this way. Imagine if someone out there found out that if you stacked Crit over Agility you could find 5% gain in DPS. Would that really make sense as a class mechanic? Sure, we'd all probably switch over for that gain, but could we really blame Blizzard for a change because we found a loophole in the game mechanics?
Now we can all debate if a 15% reduction is too much or not. (Finding that sweet spot is always difficult.) We do know that when Armor values were at 100% everyone laughed at stacking Armor Penetration. Even today it's not considered a stat worth considering until you're raiding the high end raids like the hard modes of Trial of the Crusader or at least Ulduar 25 man. But I think we can all agree that when stacking secondary stats outweigh the benefits of stacking our primary ones, then it's a problem that probably should be addressed.