Weight does add traction but it also increases inertia, really depends where the weight is that matters. You still need to turn the car which is where weight really hampers performance. Weight should be something used to balance a car not something to find traction with. People do have luck using weight to "settle" a car down making it easier to drive.

Adding weight to wheels/tires is the worst possible area to add weight in an acceleration sense. Rotational mass has a greater impact on performance than static weight in almost a 1-5 ratio. General rules of thumb say that for every ounce removed of rotational mass (transmission/wheels/tires) is similar to removing 4 to 5 ounces of static weight. (chassis/electronics)