You can do paperwork ect. in WYSIWYG, and you can't in grandMA 3D.
You can visualise blind and preview editors in grandMA 3D, but you can't do that in WYSIWYG.
WYSIWYG is used for preparation and preproduction.
With on site preprogramming and rehearsals you are better off with grandMA 3D (like in ESC 2014)
WYSIWYG and grandMA 3D have different goals. ;)