The problem is, for me and many people, how much can we remember? I fly IFR most of the time with AI HSI turn coordinator and GPS. If HSI packs up the GPS tells me heading and track if HSI and GPS both pack up at the same time, then turn coordinator and stop watch are the answer and compass confirms heading when straight and level. I can't therefore remember the compass errors and would have to go and look them up if I needed to know.