As far as climbing is concerned, lighter is certainly better, but not to the point that the frame flexes noticeably, because if you climb standing up on the pedals, significant flex means loss of pedaling power that is not transferred to the wheels. In the case of my personal carbon fiber bike, it is not only a few pounds lighter than my steel bikes, it is also somewhat stiffer laterally - those two criteria combine to make it a more efficient climbing machine than my other two bikes - but I certainly have not done any rigorous analysis to determine how much of the benefit is from weight, vs. stiffness. but my personal belief is that it's mostly the weight difference, because while it's stiffer than my best steel bikes, they are also pretty stiff too.