Do people ever use rolled marzipan as a layer between cake layers frosted in buttercream?
Probably someone has done this. Why not? On the other hand, you don't want a thick marzipan layer, because the material is expensive and has a strong taste, so it is more naturally to use the cream as the bulk filling and have a thin cover on top. This makes the cake more stable, too. If the cream serves as the frosting, it has to thicker.
Normally in a cake one often has the harder layer outside and the soft stuff inside.
As an inspiration, here is something that has marzipan on top of a jelly layer http://en.wikipedia.org/wiki/Dominostein