I don't see why you can't put durock on concrete/block wall using the proper adhesive. The main reasons for putting up a wall in the basement is to end up with a nice finish wall surface, to insulate the area, and to address moisture problems. You didn't indicate what kind of shower walls you are putting in, but if you are not worried about the finish wall, insulation, or moisture, then go for it. But then again just for a shower wall, why even put durock/cement board on concrete/block wall anyway unless you need 1/2" of fill. Just attach you shower wall directly to the concrete/block wall if you have a close tolerance. If the wall is rough above the shower stall, then skim coat it with thin set mortar and prime and paint it. We'll see if any others have some ideas here but these are mine.