The single-sided swing arm is a great idea for endurance racing, as it can allow for very quick wheel changes. But it is not too great in a typical high performance situation. The swingarm must be stronger due to the high torsional loads it experiences, and the axle and related components must be must stronger as well. If you hold a bike tire and spin it as fast as you can, while holding it, it remains stable. But if you wack it on the radial edge while its spinning, it looses stability and will wobble out of control (thus a speed wobble). double sided arms fight this by inherant design, on a single sided arm the tolerances must be exact, the strength must be far greater than that of a double sided arm. Any small amount of torsion on a single sider could end your day at the races, and with 600's putting out an average of 100hp and up its no wonder that they all have double sided arms now...