If MLEs were simply outcomes versus outcomes compared, the formula would be simple and we'd know that formula. Because its proprietary, we can't measure its effectiveness. Beyond that, an MLE is like knowing the result of a formula, the final number, and some of the variables in that formula, but not all including ones that can be large, like injuries or coaching. The number of possible variables is quite large. Someone like Casey could be a pull hitter and his home AAA park could have a high LF wall. That's just an example of a large number of possibilities. Yes, the two words aren't synonyms. Proprietary means owned. That almost always means that its done for gain or hope of future gain. The one we're talking about is probably from Fangraphs and I'd guess its a commercial, proprietary product of STATS. I don't follow the Twins closely, but I hear their fans complain about their lack of pitching including the minors. I don't think Casey will be all that valuable after one year. The people involved have probably seen numerous "one year wonders." The chance for him being a significantly lesser player greatly diminishes if he does it for another year and then there might be a bigger payoff, but that's a WAG.