A quick reply as one can write a book about this subject alone.

But the following main points are important

Stiffness allows a craft to be driven harder in rough conditions. Compare this with a car with insufficiently inflated tires. Sure it can take a bend with high speed but the flexing of the tire will give is a wobbly fell while rounding the corner. It feels like your not totally in controll. SUch wobbling can quickly cause the car to break out uncontrollably where properly inflated tires may also let the car slide but than in a controllable sense. THe improved behavoiur of the craft allows the drive to push it harder. It also feel more confortable.

Stiffness (for some reason) allows a craft to punch through waves better and sustain higher speeds here. I compare it with two hammers of equal weight one of rubber the other of steel. While both land the same weight on the nail we all know that the steel hammer drives the nail more comfortably into the wood. I guess it is the same with catamarans.

Stiffness allows such elements as the leech of a mainsail to be better controlled allowing the craft to point higher and make it more responsive to alterations in trim.

One dlophin striker less design behaved as follows : When janking non the mainsheet the mastfoot would be pushed down and the stays would lengthen together taking up large quatities of the applied mainsheet. The leech tension didn't really change that much. When the mainsheet was released a little bit the springness of the beam and stays would keep the leech tensioned up where a stiffer design would quickly have lowered the leech tension. Leech tension is an import trim factor on mainsails.

Stuff like that make stiffness important.

Wouter