Artificial depth of field tracking tends to be more distracting than anything else. The thing about how the eye works is that we switch our focus depths automatically, so whatever we're looking at is in focus. We don't notice that the rest of our field of vision is out of focus unless we make an effort to do so. As soon as we shift our focus to something that was blurry, we auto-focus and it's no longer blurred.

How's the game going to know where my actual eyes are looking? If I look to the side and the game "thinks" my focus shouldn't be there, it'll be blurry and it'll stick out like a sore thumb as being "wrong."

The only time you can get away with depth of field blur effects is when you give the viewer an irresistable "target object" to hold their attention, focus on that and blur the rest. Then the effect is really cool - until you've seen it a hundred times, look off the target, and notice the blur.