IMHO, the selling point of a thumb slug is that it offers the same feel in any ball. A typical reactive piece consists of a shell and filler, and the material feels different from ball to ball. You sometimes even have air bubbles! The thumb slug is a massive urethane (or vinyl) piece that offers a neat and uniform cover. Personally, I have all my equipment fitted with urethane slugs, I found the uniform feel to be a great benefit when switching balls, and the material is IMHO easier to tune than coverstock material.