Well, for now, if I want 6x4.5 with a decent lens, and need lightness, I wind up loading my Wirgin Auta and hoping the masks don't scratch the film (I think the problem last time they did was that I had them in backward, so the edges with rust pitting were against the film at the gate, rather than sheltered by the guide rollers). If I want 6x4.5 with a choice of excellent lenses (four focal lengths, with two lenses and a 2x tele converter, giving 90, 180, 250, and 500) the RB67 wins. The Debonair almost doesn't count -- it's a Holga in 6x4.5 only. I've got a Zenobia, native 6x4.5 folder with a good lens, light and compact, but I have to fix the case in order to carry it.
Bronica? Does the back rotate or "reverse" to give both orientations in a camera that's not made to flip on its side? Or is that only practical with an eye level prism finder? It looks like the S2 is even a bit less spendy -- what would make the S2a preferable? What about the ETRS, which is dedicated 6x4.5 -- I see a battery door, does it depend on the battery to operate?
I'd hazard a guess that the reason RB67 lenses are inexpensive is that they plus the camera you use with them are not light and are not compact.
You might be right; nor are any of the lenses I've seen fast -- f/4ish seems about their limit. The viewfinder is bright enough it good daylight, but I'd hate to try to focus it, say, inside an old barn or warehouse -- and it'll never be a "walking around" camera, unless your name is Banner and you're angry all the time.