I could understand if you are a racer, or will be riding up incredibly steep, long hills, but all this talk I see on the web about the weight of a bike or certain components is just batty. People getting advised not to put fenders or a chain guard on their ride because of the "added weight", it's just silliness.
Bikes roll. If we were hiking and carrying our bikes, that would be different. But rolling things are good at bearing extra weight with minimum effort.
And besides, riding you bike will result in its engine losing weight!