Not sure what inspired this, could be some of the stuff I was reading on the forums, but it wasn't anything to do with mistakes.
It was about the change to talent trees. It was about bad players in dungeons. It was about the removal of head enchants
So what does all that have to do with mistakes being good?
Well, people with bad specs were a good thing. You could look at the spec and tell if the person was worth taking along on your raid or not. I mean, even with the super simplified versions in cataclysm I have still saw a few really interesting, for lack of a better word, specs. People learn from mistakes in their spec. They learn why one thing is better than another. They are more likely to understand why their spells do different things if they have to choose them. Admittedly, most just look online for "what is the best spec for a hunter" and go with it, no further knowledge is needed. But for the people that actively want to learn, having the ability to make a mistake there is good.
People that are bad in dungeons are a good thing too. Why is that? Good players learn from their mistakes. Bad players might learn from there mistakes. Bad player that did learn from their mistakes become good players or at least have the potential to become good players. Mistakes are good.
With the removal of head enchants it removes the need to grind to get exalted on at least one character. But in a way, it also removes the need for people to choose their enchant or defend their choice even. Not like it was hard to choose, but I had seen boomkins with the agility head enchant because it was the only thing they could get. Okay, I can buy that, but how will the boomkin answer when I ask him about it, that is what the head enchants could teach. If they said, it was the only thing I have exalted and the extra secondary stat is better than no stat even if the agility does nothing for me is a solid answer. Anything else, is a bad answer. If people make the mistake of putting the wrong thing on, it could be a good thing.
Making mistakes is a part of the game. Sometimes you need to make mistakes to know you did something wrong. A part of the reason there are so many bad players is because there are fewer and fewer chances for them to learn from their mistakes as chances to make mistakes are taken out of the game. (and because most people choose to yell instead of teach, but that is another story)
What is next? Remove ground effects because people might make the mistakes of standing in it? Removing bosses hitting too hard because tanks might forget to use a cooldown? Remove any AoE damage because the healer might make the mistake of concentrating on a damage dealer too much and lose the tank? Remove the need to even roll for items because people might roll on things they don't need? Oh wait, they are removing that already in mists.
Mistakes are good, mistakes help people learn, mistakes are how you tell the good players from the bad ones.
What is the next thing they are going to remove to make it easier on people because they don't want people to make mistakes?
If I screw up, let me screw up. Let me take responsibility for my actions. I'll tell you what, when you have to say sorry to your raid because you messed up on something and caused a wipe you don't make the same mistake again. Well, if you are a good player you don't. And that is why mistakes are good.