I have a bit of problem shape in my heels (narrow and angular) and shoes where the heel counter doesn't adjust at all get painful with a lot of wear in a single day. Edward Green uses leather heel stiffeners, (plastic in the toe anyway) and my Dovers are the most comfortable shoes I have at the moment because I broke them in to the shape of my feet. The stiffness of the sole contributes to slipping a lot. When it softens up with wear, there should be no slippage left. Oak bark soles, as in the Handgrades, tend to take a lot more wear for them to soften up, but they also last a lot longer.