Another way to answer that is that you want both RAM and ROM addressable at the same time, otherwise you'd have no way to access ROM (including copying; at some point you would HAVE to switch at least one ROM page into at least one bank, and therefore move out a RAM page, since the four banks take the whole 64KB address space). Even if you had exactly 64KB of RAM, you couldn't make it occupy the whole address space all the time.