I imagine you want a very good approximation of the soundwave that will hit the ear (so you can cancel it out very accurately). The farther that your 'virtual microphone' is from the ear, I guess the harder it will be to figure out exactly how the sounds it is picking up will combine to hit the ear.
I mean, imagine the extreme case -- your 'virtual microphone' is (virtually) sitting in the path of some directed audio beam which won't hit the headphone wearer at all. Now you do the signal processing and generate a signal to cancel out that directed audio beam, which, because the user isn't in the path of it, causes them to hear it!
I imagine you want a very good approximation of the soundwave that will hit the ear (so you can cancel it out very accurately). The farther that your 'virtual microphone' is from the ear, I guess the harder it will be to figure out exactly how the sounds it is picking up will combine to hit the ear.
I mean, imagine the extreme case -- your 'virtual microphone' is (virtually) sitting in the path of some directed audio beam which won't hit the headphone wearer at all. Now you do the signal processing and generate a signal to cancel out that directed audio beam, which, because the user isn't in the path of it, causes them to hear it!