are you asking about the connections from the hot and cold knobs to the diverter knob? or not having hot and cold knobs and goign directly to the diverter? you would get water (hot and cold both) coming out of the shower and tub constantly without any way of shutting it off. now if you plugged the shower or the tub out on the diverter you would be able to shut the water off, but you would not be able to change the temp of the water or pressure coming out, you would have full pressure and equal hot and cold mixing.
I am not sure why Redwood says you need to use copper or IPS with many diverters (mixers) or you will get water out of the shower when you only want it out of the tub. With pex you would connect to the spout out port on the mixer with a brass threaded fitting. how or why that would effect the working of the mixer.... I do not know. I have always used galvanized for the spout, and have never used copper. Maybe I am not fully understanding what is being said here....