I use green board for this with redgard over in some cases (cheaper systems that don't overlap well). My take on it is if there is a water issue. I would rather the water be trapped in drywall and mold vs. drip down into stud bay and rot the wall or what is underneath it.
Taking out plastic sheets to repair a wall and adding more drywall is easy. The alternative isn't. Also my biggest concern with backer is the hollow sound it often projects when PVC surrounds are glued to it. Sometimes it sounds/feels like you are in a tin can.
This is all opinion though and you can go with backer or greenboard and regard or not. Local inspectors that I have dealt with don't care at all about whats behind tub kits. They are fine with putting it directly on studs. Really if installed right and caulked properly you shouldn't ever have a water issue. That is why I choose to go with what gives the best experience.