Yeah nicely put Pljones.
People set 2 channels but with disconnections and like you say people wanting to jam not mix others levels most are just constantly to others as 2 mono channels which only makes them sound worse.
As for routing as many create more than 1 ninjam channel, their tracks in reaper can be adjusted so they can hear all, un-arm and hear just what they want and even hear all but not send all to reaninjam.
So there's no need to deviate from this setup for most users unless you have specific needs.