Simply repeating "you must..." does not convince me of your position. That Buffet has been so successful over the long run is evidence that his process is the best in the business. Because outcomes are affected by so many things, it takes awhile before you have enough of them to really indicate the quality of the decisions behind them. There is so much noise and other factors that a person can make a string of bad decisions and still wind up with good outcomes. If we judge on outcomes, we're liable to both fool ourselves in to trusting people who will fail in the long run and ignore people who will succeed. Outcomes themselves are only good evidence of the quality of the decisions behind them once you have a whole bunch of them such that "luck" is filtered out -- it's not a good way to evaluate a single decision.
A good decision is one made in which the most likely outcomes are good. All that said, I will absolutely concede the point that Jocketty may have signed Cairo not for his bat, but for his leadership. We don't know what information he had available at the time. But I promise you that no information he had suggested Cairo would hit like this. Virtually any player with MLB service time could have made positive contributions. I have a hard time believing that Jocketty knew something special about Cairo other than that he could play a few positions, be a clubhouse leader, and make some contact. That he's hitting the way he is has does not suggest to me that Jocketty had some magical foresight.
Let's just agree that he's having a nice season which we all hope continues and leave it at that.