>Marc, let me give a practical ex&le to illustrate my perception of the deficiencies of the Contax 645. Imagine you're shooting candids at a wedding reception (I use this ex&le for two reasons, firstly wedding photographers are the backbone of the market from a 645 camera manufacturers perspective, without this group it's difficult to reach economic sales volumes, and secondly it demonstrates the holy grail of one camera system that reasonably satisfies most needs, medium format quality with 35mm convenience, and a flexible film or digital platform).
So after using a Contax 645 to successfully take the formal, tripod mounted shots, you're handholding the same camera and shooting two ladies engaged in an animated conversation against a background of dark shrubbery. However, in single AF mode the camera's meticulously precise but rather slow focus system may well fail to lock focus on the eye of one of the ladies as she moves her head fractionally back and forth whilst talking. Used to this problem you've set the Contax to continuous AF, but now when you've achieved satisfactory focus you have to press the Focus Lock Button (rather than simply half-press the shutter) and recompose. In the process you'll also lose auto exposure, especially if spot metering when you may over expose their faces as the centre focus & metering area is now over the dark shrubbery.
Using say a Nikon F5 or D1x (and I'm sure many others, just these are two cameras I use and I'm familiar with) you could select one of the off-centre focus areas. Far from being a "marketing gimmick" they'd work perfectly adequately in every light I've ever encountered, plus you'd get instant spot metering taken at that precise, off-centre focus point.
So, frustrated with the AF system on the Contax 645, you may choose manual focus. The flip magnifier solution you suggested isn't appropriate in these circumstances as you'd only see the middle of the frame, and it's far too slow to focus then flip the magnifier away and recompose. However, magnification is a key determinant of manual focusing accuracy, Hasselblad for ex&le offer a choice of angle finders with up to X3.3 magnification and virtual full field visibility. I don't know the Contax magnification but practical experience tells me it's not up to this level. Furthermore the Contax screen, like most AF screens, is a sub-optimal tool for manual focusing. I use the Contax MFS-1 screen and the microprism has a diameter of only about 7mm, insufficient to allow focusing for an off-centred composition, compare this to a 12mm microprism circle available for both Hasselblad and Nikon. And when I measure the brightness of the Contax screen in the matte outer field, using a remote meter from a large format camera to measure the light on the actual screen itself, I find it's nearly two stops darker than the latest Hasselblad screen, with both cameras carrying their respective Zeiss 120mm f4 macro lens. .
So our hypothetical wedding photographer taking reception candids is forced to conclude that although the Contax 645 can do the job in either auto or manual focus modes, other systems offer significantly better solutions for that particular task. The Contax tends to push the photographer into amateurish and repetitive central compositions, or risk missing the moment through equipment deficiencies.
And here's the rub, IMO that particular task is hardly some rare and esoteric application, it's fairly representative of a major part of the output of many, many photographers, from travel orientated street photography to simply capturing informal family shots.
I'm not rubbishing the Contax 645, its whisper quiet and beautifully d&ed mirror action makes a Hasselblad sound like an asthmatic mousetrap, and the exquisite Zeiss glass in the medium format roundly trounces any Leica chrome I've ever produced. What I am saying is "good" isn't "best", and Contax should divert just a little resource from their digital issues to take the 645 to the next level of development.
Regards, Gary Ferguson