You also need an antenna. Batteries are heavy, too (solar panels, like on Sojourner, might be a better choice), and a robotic arm isn't as simple as it seems. Neither is the drive. It sounds simple, but when you start considering the environment it's gonna be operating in, it suddenly stops to be. Also, every kilogram is very expensive on a mission to Mars, even more than on a LEO one. You'd usually want to use your payload capacity fully when planning the "primary" mission anyway, the missions are already taking very large rockets such as Atlas V, and the next might have enough mass for an entire Delta IV Heavy. I'd expect a rover like what you're proposing to have mass comparable to Sojourner with a MER-type robotic arm, and that's not even counting the lander hardware, heatshield and everything...