The lightweight wheels are a 3 fold benefit. 1. Overall lighter weight of the car, allows you to add lead where it is beneficial. 2. Like you said, it is sprung weight. Sprung weight effects the handling of the car, as the less sprung weight, the less weight the springs need to handle. 3. It is less rotating weight. This means faster acceleration, and easier braking.
Yes, they are more expensive, but go to the big races (shootout, Big money race, or any where else the top guys are running) and look at the wheels being used. The majority of them are running the lightweight wheels. Do you NEED to run them to win? No. But if you are going to buy new wheels anyway, it is worth the money.