The wire frame finder on my Voigtlander Rollfilmkamera and the similar ones on my Kawee Camera and Zeiss-Ikon 250/7 Ideal plate cameras appear to be quite accurate in terms of FOV and framing. Where there's larger than a peephole for the eye end, I do match up the framing of back and front; I'm pretty sure that's how they were designed to be used, since that puts the eye at a definite point both front to back and in a plane parallel to the film. With my Wirgin Auta, there's an additional factor introduced by the dual format masking in the frame finder, but even that's not hard; I line up the front and rear frames, then compose with the inner front frame (when I have the masks in for 6x4.5, which is most of the time).

One major advantage of the standard mounted wire frames on cameras with movements, like my plate cameras, or those (like a Speed Graphic) with interchangeable lenses, is that they automatically compensate for changes in lens focal length, bellows extension, and movements, simply by being in a fixed relationship to the lens while the peephole is fixed to the film plane.

Even if it weren't so dim and fuzzy as to be useless, I'd almost never use the bright finder on my Wirgin; I seldom do on the somewhat larger and heavier Rollfilmkamera (generally only when the camera is on a tripod and too low to easily use the wire frame).

BTW, I routinely use the sports finders on my TLRs, too -- scale focus and sports finder is almost the only way to use the Argoflex in dim conditions, due to the dim simple ground glass, and even with the bright Fresnel in the Reflex II it's more convenient for some shooting situations (like in a crowd or shooting over a barrier) than the waist level ground glass.