An advantage of shooting 4x5" and cropping to panoramic of whatever aspect ratio you like is that you get a certain amount of extra rise/fall, because you could crop your image from the top, middle, or bottom of the frame. Also, if you only occasionally shoot panoramic, then you can batch all your film together by cropping from 4x5" and you don't have to carry a bulky rollfilm back.
It would make sense to use a rollfilm back, if you were mainly shooting panos and wanted the more convenient/less costly processing of rollfilm and rollfilm takes up less space, doesn't require loading in the dark, etc.
Take a look at the panoramas of Art Sinsabaugh--
He generally used a 12x20" camera, but sometimes cropped to 3x20" and everything in between.