It is for the very same reasons that I don't like to advocate the use of a mouse mat and wet-n-dry for sharpening convexes (or any other type of edge for that matter). The mouse mat is inevitably going to be softer than almost any leather you're likely to use, and stands a very good chance of rounding off the edge of a knife rather than sharpening it. A firmly backed abrasive medium gives you far more control over the angle at which you are finishing the edge.