Let's say you have more than 1 zone and you want to use 2 different Prontos.

You cannot have 2 prontos connected via TCP/IP to same Marantz/Denon. And trying to use 2 prontos with RFX9600 with single serial port only leads to issues. Especially since when you send via serial port, it clears any data that might be in the buffer. Works OK for 1-way control but not for 2-way feedback.