As there ain't no such thing as a free lunch, wheels with light rims also decelerate quicker due to not being able to hold as much rotational energy as a heavier rim. Meaning you have to keep applying micro-accelerations to maintain speed - everything else being equal. Lighter wheels might help you gain time in a stupidly long uphill drag when you're at the peak of fitness, as the time taken is proportional to watts per kilo, or giving you an advantage in accelerating out of a corner in a tight crit circuit, but in everyday use you won't actually notice much of a difference to your times. Being stiff and aero is more important. The weight you can lose from your body. Aerodynamic Mavic Cosmics used to be the fast wheel of choice on a budget: they're relative boat anchors at 1900g per pair, but when they get rolling, they're pretty hard to stop.