The Porsche Cayenne is a 165-mph high-performance machine that will fit a family of five, haul a small washing machine, tow a large boat and get you carefully through the woods when there's no road. It's a 5000-pound speed-sled that can handle rugged trails.Do rapid acceleration, excellent brakes and the right sounds add up to a Porsche, or just a nice SUV that goes faster than the rest? Maybe the Cayenne is immensely practical. Maybe it's excess. Most SUVs buyers will be able to fill their needs for a lot less than the price of a Cayenne. We can't imagine that many buyers need the extreme mix of attributes offered by Porsche's SUV. But then, how many buyers actually need an SUV? How many actually need a sports car? Often, need isn't the issue or even the primary motivation in purchasing an automobile. Porsche is banking on that.