A digital camera works well for testing lighting and composition. You can also get useful exposure information from it but you need to temper that with experience, traditional meters, good judgement. But I use them all the time and for a commercial shoot I wouldn't be without one for testing.
The nice thing about instant film backs that are the same or almost the same size as your film is that you can test the actual camera for bellows factor, lens flare, light leaks, etc. Luckily we still have Fuji Instant in ISO 100 medium format size... it isn't an ideal solution but it is good, consistent film and at least you can test the camera set-up with it.
Sticking even a full-frame 24mmx36mm sensor onto the back of a ~100mm x ~125mm film camera simply isn't going to tell you much and you are going to be introducing more issues trying to use it.
I hate to say, "That's the way it is done by the pros" but it is.