But in an unbalanced circuit the common is the reference used for the signal. If there is noise added, no matter how, it can have an effect. Of course in short runs this pretty academic and should have no perceivable effect unless you have major RFI issues. That bad of RFI issues may cause problems with balanced lines also though (I once saw a system that had horrible hum, and I discovered they put their amp racks on top of a 5,000watt power drop cable. Even with all balanced cables, the equipment itself is unbalanced internally for most equipment and the amps were picking up the noise).

As far as balanced output goes, I personally don't see the need as well, although I'm curious if it might allow for higher power due to what basically is two amps working together. Certainly noise rejection shouldn't be an issue.