Its unsprung AND rotating weight... win and win. Thats weight at the very end ov the powertrain that your engine has to spin. A heavier axle costs performance, a heavier driveshaft costs performance, you dont think 8 extra pounds a wheel... spinning at a far greater diameter (thus speed), demanding far greater torsional strength is not going to cost performance? And a reason why so many teams dont go nuts on re-sciencing the rollers is because its largely been done already. Light wheels and tires are readily available, and more reasonable in price than years back. To get beyond what you see out there requires an investment far beyond return. Also, some stock car stuff is as light as its allowed to be, or as light as it CAN be. Far more stress on that NASCAR wheel than a local roundy-round wheel, or the street. You can buy 19lb circle track wheels, but the actual NASCAR ones are far heavier i believe.

If strength did not matter, we'd all be running those ugly ol' Weld Drag Stars...