Thats a BIG reason people look elsewhere for tires instead of Porsche approved tires.....People who put 10,000+ miles a year on their tires don't want to be replacing them annually, so they look for more bang for the buck.....I don't think you can blame them either. I have Pirellis on my '98, bought used, and they are a sharp looking tire for the Porsche, handle real well on dry surfaces(especially with the Sport Suspension) , but wear pretty fast, and where I live it rains a lot as well, and we know what that means!
On a dry track these would be excellent....Driving the car everyday means I'll be shelling out $1000+ a year for balancing, wheel alignment and new tires......Why wouldn't one look for tires which are a few hundred cheaper a set, perform well, and last almost twice as long?