I don't believe a player's true defensive ability can change that much over the course of two seasons. Particularly a player Trout's age. I guess it's possible but I don't believe it. Therefore, unlike ERA or BA, which are measures of outcomes that I can understand and trust, and which I believe can vary widely over a short time period, I am skeptical of defensive metrics. Having a player vary that much, when IMO defensive ability is something that can only change gradually, over time, is just more reason for me to mistrust them. I might be in the minority with that opinion, and perhaps I'll be proven wrong. Wouldn't be the first time, nor would it be a first if I'm proven correct.