"As I mentioned in Flash Player Update 3 we finally realized that multi-core CPUs are here to stay. So why not follow the times and take advantage of it? As most of you hard core Flash developers know, rendering is a huge bottleneck. I've seen a couple of blog post complaining that their second core/CPU is not doing anything when they run the Flash Player. Well people, this is about to change in this update." 
I suppose you could argue the ability to split work across multiple CPUs is a good thing... however, this is just a sign that they really just need to start pushing some of this work to the GPU instead of taking up more and more CPU power (both Silverlight and Flash will be using the GPU to some degree with video... why not push a little more down the pipe?). One GPU can do far more rendering than a couple CPU cores, and the gap is only going to widen in the future. I can see how multi-core rendering might be useful, but I actually like the fact that on my multi-core machines the processes that are going bonkers are capped at 100% of one core. That means I still have 50% of my CPU power available to make sure the rest of my machine is still responsive and I can still click on buttons in other apps. Are Flash movies really getting to the point where they need to suck up this much juice? Looks like with the next update of the Flash Player, an over the top Flash movie will once again be able to drag down the performance of my entire machine.