Reducing the friction co-efficient of the rubber and contact surfaces of the wheel results in a "faster" ride. Reducing the friction in the bearings and hub along with a lower RR in the tire means that less power is needed to rotate the wheel. Thus producing a quicker rotation of the tire.
Even if a lighter wheel was used the contact oval shaped patch of tire to road would not change. Assuming no fluctuations in inflation. The oval would not change because the weight of the bike and rider remain constant which creates a "standard" oval. Lightening the rider and the bike would result in minimal gains of decreasing the contact oval.
Glad to know that I was looking in the right area of the world when I was looking at replacement wheels.