The screws wear down, too. If you don't take them out soon enough, they can grind down so that there are no corners or central slot for the screw driver. Once removed, they leave a small mark that does not affect the shoe's performance. My guess is that the screws actually save on outsole wear. I've never tried running icy trails with them.
Slick pavement is no problem, though.
On dry pavement, you might hear some irritating clicking or scraping noises. And be careful walking indoors on the linoleum. I drilled a small hole through the outer sole, so that I could get the screw started. I put eight in each shoe around the outer edges. It is important to put two in the toe area as well as two in the heel area so that depending on how you plant your foot you are safe.