If you read the rest of my post, you saw that I acknowledged that pitchers with a better defensive behind them tend to have lower ERAs. However, I don't believe that discounting ALL defensive plays is the way to accurately measure this. It's like saying since some QB's have better receivers than others, we are going to completely discount all attempts and completions when assessing QBs and only count times that they score on there own, since that's all that they can truely control. It doesn't remove that component. It sets it to league average, or a neutral environment. If an average defense allows a BABIP of .300, and a pitcher has a .330 BABIP, it removes that generally uncontrolable factor. xFIP is a simplification, definitely not perfect, as pointed out already. But it does a good job of removing luck, natural variation, defense, and park effects. Those are not the only factors that influence ERA, however, but they are large ones.