If you are going to use a surface waterproofing, then I suppose you could use the Kerdiband there, otherwise, I do not think I'd use it at all. With the plastic behind, and Kerdiband, any moisture that may penetrate the cbu would be trapped. The beauty of a surface membrane is nothing on the entire wall behind it gets wet. In what you have, the cbu can wick some moisture, but the plastic film behind it protects the studs. The tiling flange (if your tub is level!) keeps water from sloshing into the walls from the rim, and the tile prevent major moisture from getting to the cbu in the first place, so it only has to dry a little, since it never gets very wet in the first place.
Now, if you wanted to use the Kerdi system and waterproof the walls, Schluter has some diagrams on their website that describe how to do that. You'd use some KerdiFix to seal the Kerdiband at the tub wall junction, and thinset on the Kerdi/Kerdiband to waterproof the seams going up the wall.
There are probably some hybrid systems you could use, but you wouldn't have a warranty. Lots of tub/showers work and last with the plastic sheet draped over the tiling flange, tile and caulk at the tub/wall junction. As said, though, if the tub isn't level, and water pools or flows back rather than into the tub, those can be problematic.