Because its cheaper and Grandma doesnt even know what an operating system is let alone the difference???

So your comparing a dual 2.0GHz 2+ year old system with 2.5GB of RAM against a quad 2.66GHz new intel system with 6GB of RAM and your blaming the OS for making the G5 seem slower? 10.64GHz > 4GHz the last time I checked. Leopard must be slower than Tiger, if you believe all the performance advantages of your new machine being twice as fast are negligible.