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.
