Let me be the troll;
Really? I think thats Apple propaganda, as everyone has to read that the OS is not afflicted by a application crash for years now. I have some reale stabel OS 8.6 machines that never need reboots at all. OK, PM could be nice but it is not mandatory.
I do not need it. In fact cooperative multitasking has its advantages. For example I will never see such a snappy GUI at any X or Debian or whatever. It drives me crazy to see how fast a OS can react at 1 GHz when I use the Quadcores with Linux, ...I belive cooperativ multitasking is better for desktops and single user computers. Applications like Quicktime, that grab everything and block the entire computer are a pain – that´s true - but this is a application problem and not one of the OS.