Ian K said..
Why is boom stiffness so important? It's been said that when the boom flexes in a gust it gets shorter, powering up the sail when you don't want it. When you do the boom flex test, do it with the head firmly on the floor and the the tail against a pencil mark on the wall. Note how little flex is transmitted to the tail end for quite a bit of flex between the arms. It's all simple geometry. Aluminium has a nice comfy feel and if you do get tangled with it in a big crash it's more likely to progressively crumple doing less damage to you.
For aluminium booms, the stiffness translates to durability.
YM for Al is around 70 GPa. Carbon fibre in epoxy is over double that. Aluminium is at a disadvantage here, it's easier to flex it, and as it flexes, it becomes more fatigued (in alloys, strain is a good predictor of failure), and more likely to fail.
As for comfort and crash absorption, yes, agreed it's better to have the flex.