It would be the same as the Master option, but non-mirror masters would not be able to exchange changes between themselves. The shared folders everywhere would be identical to the folder on the Mirror Master at all times (unless a non-mirror master makes a change, but then syncthing would correct that). Plus no more annoying red “out of sync” button on the master!
I found this issue: https://github.com/syncthing/syncthing/issues/62
Yeah, I think that issue about sums it up.