I think a pad for an optical mouse is definitely worth it. For one, those little pads on the bottom of the mouse DO wear out. A desk isn't exactly the best surface to scrape a mouse across, getting a pad will reduce noise and friction. Less friction means the mouse will move faster
Most pads also have a somewhat geometric pattern on them, also, which will keep the mouse tracking easily. Not much of a problem now since Logitech has a massive frame buffer on their MX mice, but the concept still holds.