I needed one for my glass top desk. But, I just bought a different type of pad instead - the Magic Trackpad🙂
That said, a mouse pad is needed for the Magic Mouse if you're like me and are OCD about scratching those nice and smooth black runners on the bottom. Looking at the ones at the Apple Store, you'd think people were using it on sandpaper...gross...a mouse pad provides a soft surface for the mouse to slide😉