Shouldn't pressing the Up and Down keys while the program field is focussed do pretty much exactly that?

It does, but that requires moving keyboard focus to the program/bank spin controls which means having to constantly switch focus between those controls and the note input field. I don't know if using the mouse causes the note input field to lose focus, but with my screen reader I have to switch focus between the two sections so it is difficult to change program/bank numbers quickly while still having the ability to play notes.