you're right that more weight = bad

but there is a difference between weight added to the rotating elements of a vehicle (ie: wheels/tires/driveshaft/flywheel) and just adding weight to the vehicle overall.

obviously keeping the weight down will improve nearly every aspect of the vehicles performance... however when talking about weight in the drive train that weight dramatically effects acceleration... in most vehicles 1lb added to the drive train effects acceleration and deceleration as much as adding 10lbs to the overall vehicle weight. This obviously varies depending on the drive train but the point is that keeping the weight down in the wheels and tires is MORE important to performance/economy than just keeping it down.

That's not even talking about the benefits to handling characteristics. The suspension works better with a lower amount of "unsprung weight" (eg: the amount of weight on the rubbery side of the suspension mechanism).

It's all about momentum... it takes more effort to get heavier tires moving or stopping... be it rotating or bouncing up over a bump in the road....

It's why racers swap out their heavy flywheels and steel drive shafts for light weight aluminum versions... they might only save 50lbs to the overall vehicle weight, it doesn't add any hp or torque but it changes the acceleration characteristics as if they'd dropped nearly 500lbs off the total weight.