Maybe, but the question is why you would do a less accurate approximation when you can easily do a better one? For example, before true HDR lighting in games w

Maybe, but the question is why you would do a less accurate approximation when you can easily do a better one? For example, before true HDR lighting in games was a reality, Half Life 2 faked iris adaptation by placing a brightly colored quad at the end of an exit from a dark tunnel into an outdoor space, and fading it out as you walk toward it, as if the player's vision is adapting to the brightness. Now the same effect is achieved by rendering to a floating point buffer which has much more dynamic range that can be displayed, and compressing that range in post-processing with parameters based on the light levels of the area the camera was focused on in the previous frames. It's much more faithful to real life, and it's less work than the faked solution from Half Life 2. You might be right that AO adds value beyond enhanced "realism", but it could also easily add to a long list of graphics techniques which look jarringly bad when compared to the more refined technique which replaces them.