I agree. I think defense is under valued. Not knowledgeable in these advance stats, but let's take an overly simplistic look at two outfielders. Now most fly balls are pretty routine and every ML quality OF is going to catch them. But once every other game or so, there will be that one fly ball that the elite defender, player A, is going to track down that the subpar OF, player B, is going to let fall for a base hit. Maybe that is a bit of an exaggeration, but I don't think the frequency is that far off.
So player B gives the other team one additional hit every other game. What does he have to do offensively to offset that? Simplistically, he would have to have one more hit than Player A ever other game, which would translate into a BA of about 100 points more. So I can see how an elite defender batting .230 is more valuable than a subpar defender batting .310. Now if he is also hitting 40 HRs, that changes things. But it takes a huge bat to offset defensive deficiencies, and I don't see that in how most teams value players.