Garrett Hack replies: One of the advantages of cast iron plane bodies is that they hold up well to the wear and tear of abrasive wood. I suspect that the new ductile iron from makers such as Lie-Nielsen is harder and tougher, but the wear problem is not eliminated.
Soles abrade and miles of shavings wear the throat open. For fine cuts and accurate work, the sole has to be flat, so the plane doesn’t rock around. I want a jointer flat at the toe, heel and at the mouth, so that the whole length of the plane guides the cut. The same is true of my shoulder rabbet plane and smooother doing finish cuts. For a #4 doing rougher work, flatness is less important.
My well used cast iron planes need a minor touch up on a sandpaper lapping plate once a year, or more likely every few years. My wooden planes get this treatment far more often.