Maybe some influential writer or publisher (like the Bridal Bargains book) got confused about this at some point and then the error has just been replicated over and over again from there?
Thing is, ordering kitchen cake CAN save money -- but not if you have an elaborate dummy cake too! The "standard" way of approaching this when I got married (many years ago) was to have a smaller tiered cake than you actually needed for the number of servings, and make up the difference with kitchen cake(s). There is NO dummy cake involved. Under cakequeen50's pricing scheme, for example, a four-tier decorated cake plus a simple kitchen cake WOULD cost less than a six-tier wedding cake with the same total number of servings. For a big wedding it can be a significant savings, and the guests can't tell that an individual slice -- showing a one-inch wide strip of frosting -- came from a kitchen cake rather than a tiered cake.
Anyway, for those of you with brides asking about this, you might try explaining it that way.