Why so light? (driver and wood shafts)

As as has been said, light shafts allows players to generate more speed.

My Motore F1 is 55g, which is very light, but is stiff flex. It suits me as I generate good clubhead speed (av. 106 on a monitor) but don't go after it... I have a fairly smooth swing. I get more out of it by thinking about rhythm, rather than lashing at it. I just forget that fact somewhere on my backswing

Poulter was trying out a 55g shaft in his ZL Encore over winter (Twitter geek alert), so it goes to show that there is more to the equation than stiff and heavy for fast swings.