For me, it's VCS friendliness. Try checking a .fcstd file into git and see what changed between commits.

Also, it's extremely reassuring to have a description of your model that wasn't generated by a program. In earlier versions of FreeCAD, my model would often not be saved correctly and be totally broken when I opened it again. FreeCAD also crashed randomly on me quite often. In contrast, all you need for editing .scad files is a text editor, and that probably won't crash that often. :)