If your showers are so perfectly waterproof, who gives a crap if there's drywall behind it? It's only function is to provide a stable surface to attach the waterproofing to. Drywall is an acceptable substrate for backsplashes and other tile applications, so reason would state that it would be a sufficient substrate for a shower, if it was water tight. I thought your waterproofing was perfect, so the drywall behind a waterproof shower would actually be the best place to install it, as everywhere else, its exposed to a higher level of moisture and has a higher chance of becoming a mold problem.
That said, I'm not advocating it as a substrate behind anything that it is not specified as acceptable by the manufacturer. Kerdi specifies it as an acceptable backing. It is not acceptable to use as a tile backer w/o waterproofing in a shower, which would be a "wet zone"... can't tell you how many times I've seen that... "Ceramic tile is waterproof, right?"