Why are roadies concerned about weight?
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.