this is simple.

when i am bored of a conversation or do not find interesting i can look at the background to find different details. i love this with kubrick movies like clock work orange or with the usual suspects. that when you pay good enough attention you can figure out clues. remember star wars episode 3 when E.T. was on the background. there is lots that you can do with background if you actullay want to. i love this in the orginal blade runner as well. there is so much detail in the background.

if your a good director you give people the freedom of choice what they want to see or not. but when you focus only on 2 faces or 2 half bodies and blur our the background complete there is nothing left to see. your destroying the scene. then why not just add a blank screen behind and be done. because in real life when i am bored of a conversation i also start looking around me.