My point about light bikes is that the difference when climbing/accelerating and so on is very easy to measure/calculate – save 5lbs on a 25lbs bike and the bike itself will be 20% easier to get up hills. Add in a 170lb (just over 12 stone) rider and that drops to 2.6% – definitely easier but not all that significant compared to other factors (plus saving 5lbs isn't that easy/cheap!) which can have much greater effect and are well know to fluctuate by a fair amount day to day – eg hydration, rider weight, fatigue, nutrition, psychology etc.
Of course, if you had a 175b rider with a 25lb bike and then gave the rider lipo, taking out 10lbs of fat and then attached those 10lbs to the bike instead of the rider, the rider would complain that the bike felt heavy and slow despite the fact that he would almost certainly ride up the hill/accelerate and so on exactly the same as before – that's the psychological difference – light bikes DO feel nicer to ride even if they don't inherently make anywhere near the difference that the rider might feel they do.
There's also a lot of confusion about whether it's the light weight or the bike design that makes it feel good – bikes that are light tend to be designed to transfer power well/be efficient so it's easy to confuse reason the bike feels good when climbing/putting the power down, attributing it to a couple of lbs of weight saving rather than a stiff frame/wheels/etc with fast tyres and so on.
