Ya, it's definitely pretty rare/niche for the down simul to be the right/best option.
I'm thinking long alpine where no established rappel stations exist and you have a short rope (so rappelling and leaving gear is costly time and gear wise), but terrain generally well within capability ... short sections in ridge climbing where you need to go down or around features with a downword trend ...
Just exploring to see if something was out there, and i think not really. Maybe revo would give at least a (false) sense of safety, and maybe better than nothing.
Alternative is to just down lead/follow with proper pitches (at the cost of time, vs being able to simul). In that case it's pretty standard and not THAT much slower.
The example that sparked my interest was a linkup at Tahquitz, we went down the Trough. Could rappel, but no real stations, and our 50m rope would have needed probably 3-4 rappels, plus the lost time to transition, plus being in people's way for longer, etc. We down simuled in two blocks, no big deal, but got me curious if something existed (a la the trax used while going up), to engineer a little more margin, hence the Q&A.
Thx again for your thoughts, I may pick up a revo just to test it out for another tool