Does the type of sheet rock behind a glue up tub surround matter?
Although the material may be covered there is still a potential for water or humidity to come in contact with it. A small leak may go unnoticed  until mold or rot are evident. The water resistance is dependent on a seal whether it is chaulk, grout or some other material. Water resistant materials are chosen for their "just in case" or "what if" properties. Think of it in terms of your roofs. Shingles alone may keep the rain out but it is comforting to know that the ice shield or roof felt are backing up the shingles just in case.