What is the practical advantage of using mirrored signal outputs eventhough what matters is balancing the wires?
As has been mentioned by peufeu, if you use balanced differential drivers you create twice the p-p voltage and hence the SNR is doubled because noise can be assumed to remain as it was previously. But, there is another subtlety that was alluded to in the question and this relates to impedance balance.

Quite simply, if the balanced driver had an output impedance of 20 ohm (ignoring the output feed resistor R5) you would need to add 20 ohm to the balancing drive resistor (R6) that connects to 0 volts to keep balance.

But it's a bit harder than this because the output impedance of a driver chip will likely vary across its bandwidth so, it's much easier to use a dual inverting driver and equal values for R1 and R2 (right hand diagram).