Depth of field... how's it done?

You're thinking is very abstract. Try looking at some movies to see if the use of lenses and photography enhances the experience or not, or what the scene would look like with different choices. From this standpoint direction and photography would be totally useless as well. Deps made a good point with Myst 4, since as you were asking the focal point is actually where the player is looking, but even in a different case your point is feeble to the point that I would wonder why directors bother otherwise. Not necessarily the author wants to replicate the human eye but more to convey an emotion.

After all we could extremize this line of thinking as to go back to filled polygons, as everything else is beyond need in order to understand what you see on screen.

I for one don't like improper use of effects, but you can't condemn the need to have advanced real life effects, expecially because even though we are faking it in real time, focal point is the base of photography (and since lenses are already mathematically replicated with lifelike detail in rendered graphics we can expect to get there in a few years). Anyway, lens-flares as well would be great if recreated realistically with lenses parameters instead of being emitters attached to a body... the possibilities are far different from the effect you saw in the games of the 90's.