Why are roadies concerned about weight? I have never ridden a road bicycle, so this question is probably silly (and a duplicate). Still:

Assumptions:


Road bike
Riding fast, 30km/h nominal
Few and gradual corners
No abrupt stops (i.e. no traffic lights)


In the face of those assumptions, why are road cyclists concerned about the bicycle mass at all? The large inertia of a heavy bike should be no problem and should even be providing stability and comfort. The only conceivable drawback would be higher friction in the bearings, which should be negligible.
There are two main reasons: 


Hills: Pushing a light bike up a hill is obviously easier than a heavier bike.  Everyone likes to brag about that HC climb he did the other day.  Without resorting to lying about actually climbing the hill, the easiest way for a cyclist to make his life easier is to carry less mass up the hill.  Nobody will say, "That didn't count because you rode a 3kg bike," and you get no extra points for lugging a 40 kg dutch commuter bike to the summit. 
Technical Competence: Bicycle manufacturer have tried over the years to reduce bicycle weight, to the extent that weight (or lightness) has become a proxy measurement of the manufacturer's technical competence.  Anyone can build a heavy bike, but it takes real engineering skill to build one that is light.