Well, I'd say that overall, weight of any kind is undesireable in a car.
Unsprung weight is that material at the four corners of the car (generally) that is not supported by the suspension elements.
This includes wheels, tires, some fractional part of the suspension, along with the axles, brakes, hubs, etc. A reduction in this weight is generally regarded as helpful in keeping the tire patch in compliance with the road surface, and less dramatically subjected to physical forces acting upon it.
Carrying these ideas to ridiculous extemes caused folks to dream up such things as inboard brakes, and to have GOOD ideas like alloy wheels (and alloy wheel nuts).
