*headeesk* I guess this thread proves it's not the customers with the 'sheet cake' mentality, it's the decorators!
If you offer sheet cakes for less than any other kind of cake and think that you gotta spend less than 10 minutes slapping icing on them, then YEAH, they ain't worth it. But that's ANY cake. Sorry, but you can just slap icing on a couple of tiers, pipe some crappy border around them, and crank 20 of those out a weekend too. (I've seen some at Safeway once in a while)
If someone wants a unique, one-of-a-kind, special, yummy cake with custom decorations that will take days to make, why is it a slap in the face or "less than" if they happen to want the cake as a 9x13 rectangle instead of a 8" round with a 6" tier? A cake is not more special if it has another cake stacked on top of it.
Wal-Mart cakes are Wal-Mart cakes because their decorators are only allowed 5 minutes to decorate a whole cake, not because of their shape. And Wal-Mart WILL do tiered, drapy, stacked cakes too. For $125