I'm talking about usability, not sound quality.
I'll give you an example. In many situations I use the crossfade of my OT to kill the lows of the Kick and because the bass line comes from the A4 mixed with the rest of the synths, usually the lead synths, etc., I don't want to cut lows on the OT THRU machine because it will be killing lows on the lead synths also.
Because of this on the A4 I have to use two kits/and or to program performance knobs to change bass line second filter to High Pass (I usually use multimode filter on lowpass for strong bass lines), then rise the multimode filter frequency, and after the music break, put all back to get the bass line's lows again.
If I can send the base line using an extra output I will use another TRHU machine on the OT with a highpass filter controlled from the Crossfader, and I won't need to take care about driving extra knobs.
I can think in many situations while playing live where you may want to process or resample A4 stuff on the OT and it will be very convenient to have them separated.