The manual for my a corner neo angle shower i bought says to install the base right onto the studs and not over any drywall. I was always under the assumption you drywall the back with humidex/aquaboard drywall.. So i guess they want the drywall to start 5" up or so to attach the shower surround.. but why not behind the base?
the only time i see drywall installed behind a shower is when there is a fire rating for that particular room. Meaning in an apartment or condo the back wall is connected to a hallway or is the median wall between two units otherwise i think its really not need