I've tried that, using various brands of margarine, and found that buttercream made this way is really easy to spread on a sheet cake, but not stiff enough to pipe on decorations.
I did one for my son's bar mitzvah a long time ago. I made a sheet cake for the body and two 9x13 sheet cakes for the scroll parts. Each 9x13 was cut in half lengthwise, stacked, and carved. The ends stood out a little above and below...