Originally Posted by kingfishr

It's all about inertia, air-resistance and friction

The question is not new... Triathlete Magazine 1999

We have two riders competing against each other in our hypothetical time trial.
The first is Mr. Heavybike, whose bike weighs 22 pounds.
Then we have Mr. Lightbike with a 17.6-pound bike.

Both riders are identical, each weighing 165 pounds and able to produce 250 Watts of power in our time trial. Both bikes are also identical except for weight. Both riders have equal aerodynamics. These are typical values for riders who can do a one-hour 40K.
If you look at the physics, the weight has no input into the calculation of drag on a flat course, except in one place: Rolling resistance. This is the drag created by the tires against the pavement, and it is really heat generated by the rubber as it deforms across the contact patch. When we do the math, 250 Watts will propel Mr. Lightbike at 24.99 mph, and Mr. Heavybike at 24.96 mph. That gives Mr. Lightbike about a 3-second advantage in 40 km. When was the last time you lost a race that long by three seconds? How much is that three-second gap worth to you? That’s the consideration when you pay a bunch extra for lighter parts, and only you can assign that value. But remember that we are comparing bikes that are 4.4 pounds different, not 4.4 ounces.