For studio cameras, I would assume they were used often. You could set up one camera and change the back depending on intended usage. Deardorffs also had sliding baffles to get multiple images on one sheet.
In the field one attraction of a reducing back is that by using a smaller format on a bigger camera, you reduce the problem of bellows flare (which can also be reduced by use of a compendium shade). It also lets you use all your lensboards on the same camera or go to a smaller format when you don't have a long enough lens. You might also want to use a larger format for B&W to contact print and a smaller (and less expensive) format for color to be enlarged or scanned.