I think its more a confidence and strength thing, they are not a technical movement. If you can do a decent looking pull with something above your clean or snatch for a couple of reps then you have a bit more confidence that you are building towards lifting it in a snatch or a clean. If you are doing lighter pulls, at power snatch or clean weights, then you can replace them sure, but that situation would only arise if someone had a wrist , elbow or shoulder injury I imagine.