When all the recordings are made, the final mix down is done into the DAW with all the bells and whistles it can offer (including a full stack of effects applicable separately to each channel). To say it all, probably the few using the Genos & Co as an expander will only send a single channel at a time (and record back the audio into a DAW). Who realistically needs 32 input midi channels (= a jam of 32 different instruments) mixed together by the keyboard into a single audio stream? Especially if you consider that Genos & Co can only mix down all the incoming midi channels into a single analog audio stream, output trough the onboard speakers and/or analog audio jack. In this use case 16 midi channels are usually more than enough.

The midi in is needed when you use the arranger as a sound module ('expander') driven by a PC (sequencer). In this use case midi in/out is never used. Yamaha arrangers have 2 midi ports in order to allow to reproduce a midi file with the internal midi player (that could saturate 16 channels) and at the same time allow the user to play in real time above the midi file playback (another few midi channels needed) using the built-in keyboard.

How to open yamaha usb midi driver