When I got my first Factor pedal (PitchFactor), one of the first things I found myself wishing for was S/PDIF i/o so as to allow for more potential interfacing options within my recording rig; when I got Space it became even more desirable as I would've loved to have been able to have the option to slot it in somewhere either amongst my rack units (most of which are connected via S/PDIF already)…or as far as direct connectivity to/from my computer's DAW software via my soundcard's S/PDIF I/O.
In my own case, it's as much as desire to keep as clean a signal path as possible as it is to get that "differences only a dog can hear" kind of thing ….I've found the more equipment I have connected via S/PDIF, the less likely my signal seems susceptible to environmental stuff like 60Hz hum, RF interference, etc.
Being able to pass audio digitally between Factors would be a welcome addition. I agree, too, that my dog probably couldn't hear the difference (he's likely got hearing damage already, though). To me, this would be more of a concern in recording usage, but not so much in an amp loop (or in front). Still, I'd prefer to keep A/D/A conversions to a minimum.