By providing a L/R audio balance option, a single computer can play 2 streams at the same time by simply separating the speakers. Putting the slider in the player's advanced menu would keep it out of the way until its needed.
As stream viewing becomes more popular, its just a matter of time before 2 streams want to be watched by viewers but only 1 audio card is available.
Also, if it doesn't require extra processing, adding a mono option could help the auditory handicapped.
Cheers!