I question this...

In my experience riding in wet weather, the wheels splash road grime on the rims, especially the rear rim. This grime contains sand and grit. When applying the brakes, some of this grit gets caught by the pads and end up embedded in them. It is this grit which is responsible for the majority of the wear. Seems to me softer pads would allow grit to be embedded even more than firmer pads.