I feel like a dummy asking this...but what exactly are dummy cakes?? Are they syrophoam tiers covered in fondant??
They can becovered on Fondant, Buttercream.
Some decorators use them to practice on. A lot ofBakeries use them to display their work. I'd assume they would last a long time, unlike a real cake.
Some brides use them to cut the cost of the cake and still get that 3-4 tier cake that they want.
Probably 99.9% of them are styrofoam but........they also can be created by covering the pan, piling up cake boards, molding rice crispy treats etc etc then treating them as a cake by covering in any icing medium you want. Almost all the ones I made were iced in b'cream