no real explanation needed when you think about it, Ted. The parts are all piano. Can you hear any pianos you are not expecting? No. Therefore these parts are not being used in technics auto play or easy record. Simple as that

The Part 4 should be used for technichord because it gives you a third voice in live right hand playing, and even if you don't use the third voice it gives you the opportunity of balancing the harmony sound. Full details were in the 6k book, with a precis in the 7k book.

If you want to multitrack you use these parts, and that was described in the 6k book too

Load a general midi file and look at the sequencer play page or the mixer - that's how these parts are used!