I do get benefit from the interchangeable backs - in two ways.
1) I do from time to time use both colour and black and white film for the same subject. The ability to switch mid-roll greatly increases flexibility. Of course, it also increases the likelihood of being left with one or more half shot rolls; and
2) the part of all these older cameras that is most likely to require service is the film transport mechanism. And interchangeable backs provide a good portion of the benefits of having a backup camera.
Two Mamiya 645 Pro shots from the same outing - one TMax 400, the other Ektachrome:
1) I do from time to time use both colour and black and white film for the same subject. The ability to switch mid-roll greatly increases flexibility. Of course, it also increases the likelihood of being left with one or more half shot rolls; and
2) the part of all these older cameras that is most likely to require service is the film transport mechanism. And interchangeable backs provide a good portion of the benefits of having a backup camera.
Two Mamiya 645 Pro shots from the same outing - one TMax 400, the other Ektachrome: