Like Craftsman tools, there is good, better, and best. There are many new homes being built with enclosed tubs and standard drywall is put up in the bathroom. Better method is to put in moisture resistant (greenboard) in bathrooms. Best for resistance purposes is durock but really durock is best when wall tile is being put on. That said, there are still some people who put tile on greenboard. With a fiberglass surround, I would use greenboard around the shower for sure and since a bathroom isn't very big, I put it all the way around including behind the sinks and toilet. Greenboard is heavier, stronger, and more expensive. However, greenboard feels more solid and when you go back to installing regular drywall, you will notice the difference when dropping it or setting on and breaking corners, etc.
Someone one could do it differently, just like buying tools, do you want good, better, or best? It depends upon the use.