I pretty much routinely use the magnifier and hold the camera up with it to my eye when I need to focus critically and don't have any problems with that. YMMV.

I'd think a prism on a TLR would give up something, somehow. It's not, I think, that the perspective from waist level is that different, it's more that the right angle viewing adds an element of abstraction that seems to aid composition to me. YMMV, maturally. Of course WLFs are available for other types of cameras though they really only make sense to me with a square format.