What's up with all the "single lenses?"

There are hardly any cameras left that use an auxiliary lens for the viewfinder. There are no new twin lens cameras, and even the optical viewfinder like on the high end compacts is a rarity. There's really no distinguishing meaning left in the "single lens" designation, is there? I think the point that the lens is interchangeable is far more important, and this feature may be obscured by the "single lens" name, especially when without the "reflex" suffix it's not clear what the big deal is.

Vlad