Actually, this is a pseudo-FAQ, since no-one has asked any questions yet ;-) !
OK, so why don't we use an AVR instead of the Z80? Well, this would lead to other problems. Some AVRs can support external memory for data. This means the 'ROM', i.e. the language would be fixed on the AVR so essentially we would be forced to run an interpreted system. For example, we could support a p-coded language or Forth. The problem then is that the interpreter would be slower than native Z80 machine code (though it would be much faster than p-code or Forth on the Z80).
Video generation would be less problematic, because it'd be easy to formalise comms between the two AVRs so that the CPU-AVR would know all it had to do was generate addresses for the Peripheral-AVR; we can keep those in sync easily.