So now I know! OK, I wasn't thinking about the cost - with 35mm, that is negligible but I understand that with larger formats it will get expensive.
Not so much about the cost. When you test on film and you got good exposure are you sure that your shutter speed is correct? May be you would say you don't care as long as you get good exposure. What if you get bad exposure how do you know it's because the shutter speed?