The thing about using butter in buttercream is that it isn't as safe to leave out for a long time. In 4-H, kids can't use butter in frostings when taking to the fair. The shortening recipe is the one they have to use because of food safety. Several members of my family once got a bit of food poisoning after eating a birthday cake that had set outside for several hours (butter in the frosting). This is something to consider when making a cake to sell.