Well, I'm not an expert in this area and I have no intention of getting into a debate about the science, but it is my understanding that tires with a smoother tread have better grip (particularly while cornering) because of their increased contact patch. I understand that in the ideal circumstances represented in basic classroom physics the frictional force is independent of the surface area, but in the real world of bike tires and pavement this isn't entirely true because there are some properties of bike tires and real world pavement that breach the assumptions made by the classroom model.