I honestly have no idea why anyone would add it, ever. The whole concept of depth of field is in real life if you focus on something far away then other stuff becomes blurry. That is great, but I'm focused on the thing I want to look at and it's not freaking blurry the crap I don't want to see is blurred and who the **** cares. Games do not have eye tracking, I hope they never will because what a damn waste.
It's NOT realistic, it's unrealistic, it introduces a situation that simply doesn't happen in real life, that is, the area I want to look at directly with my eyes doesn't come into focus.
I can understand it's use in video even if I don't really like it. In a cutscene or film then the director/whoever is making it can choose where the character is looking and attempt to make the focus real... but it's over the top and stupid. There are some games with epic areas, maybe a cutscene in space which blurs out all this really cool looking crap to get you to focus on something which may or may not be interesting. I still would prefer the player gets the choice. But in actual gameplay, not letting me choose where to look and blurring out stuff I'm actively looking at is beyond retarded.
You can't know what I'm looking at so stop attempting to blur other stuff out... not least because if I'm really not looking at it, it doesn't matter if it's blurry or not, if I am looking at it, it's incredibly important it shouldn't be blurry.
Even things like looking down ironsights, doesn't matter, in real life I can put a gun up and look down the barrel.... and still move my eyes left and focus on something entirely different.