I tend to agree with Metakirby's explanation for the outrage over this card. And I would go a step farther and say that a card like Purify seems especially bad because various other class cards from the new expansion are so clearly likely to be useful. Some classes are getting cards that give them an obvious boost, and that's great. It's just that Priest needed a boost more than others.
And Matt's right--there are worse cards than Purify in the game. It's not the worst card ever. Though I don't know why Bliz didn't just allow the card to silence -any- minion. Then it would be useful, and at 2 cost, feel like a real boost for the class.
As to the statistical point that's being quoted everywhere from Mike Donais about there being only a 5% win difference between the best and worst classes, that stat isn't exclusive to ladder play, nor is it exclusive to standard. As stated, it includes Wild and Arena, and I wonder if it includes Tavern Brawls as well. I'd like to see a stat for standard ladder play-only win rates.