On the flat, weight only affects you during accelerations - at a steady speed, it has no significant impact on performance. Aerodynamic drag and rolling resistance are constantly sapping away power, so features that reduce these losses are nearly always worthwhile even if they increase weight. Even on a moderately hilly road stage, aero trumps weight by a considerable margin; on the track, weight is almost entirely irrelevant, particularly in longer events.

A lot of riders like the feel of a lightweight bike, a lot of them believe that light bikes are faster, but that's only true on exceptionally steep stages or hill climbs.