For decoration purposes at the wedding either to stand in for the real cake (which will cut up later and may just be covered very simply in icing to eat) or to adorn a dessert table or just to provide the look of extra cakes on the same table for an elaborate and fancy look.
ANot often, but sure.
AI only get this request when a bride has read in a magazine or on a blog somewhere that this will save her money. I do a complete break down of the numbers for her and explain why that is 100% not true.
I do occasionally add a faux tier to a cake to make a larger overall cake without adding extra real cake that has to be eaten.
ASometimes a fake cake is hired along with faux centre pieces and even the bridal bouquets then wheeled out the back door via the kitchen where a cutting cake is being prepared for guests. Hiring venue decorations and cakes is becoming more acceptable as brides want a luxury look for photographs that can be recycled by a hire company.
One of my SILs had a faux cake with the sheet cakes out back being cut by the caterers while everyone oohed and aahed over the faux cake.
I never said anything to her nor her husband...I stood next to the faux cake the entire time because it was a hot day, it was covered in rolled buttercream with fondant and modeling chocolate accents, and it was starting to bead with sweat (we were on a deck overlooking a lake). I was so petrified everything would melt that I just stood next to it and prayed to the cake gods :D I didn't realize it was faux until I saw a bit of styrofoam peeking out from the seam on the bottom layer where the border had melted enough to pull away.
It was a lovely looking. My SIL had a seashore theme going on, so there was seafoam and sea fans and shells and a baby crab or two and sanding sugar sand and all sorts of neat things all over the cake.
IIRC nobody made a big deal that it was a faux cake. It was the first time I'd ever seen one "in action", and it served its centerpiece purpose.
I have also gotten requests occasionally for a fake cake. It's interesting to see their reaction when I explain that the majority of the cost of a wedding cake is from icing and decorating, and it takes me just as long to ice and decorate a styrofoam cake as it does a real cake. Unless you find a company that specializes in that sort of thing, since they would be getting multiple uses out of a single cake design, it wouldn't save them any money.