Well, crunching and folding are both DC projects (distributed computing) and are both based on a single principle. you get a WU (work unit) from the internet, and your machine processes it and sends the results back to the server, again on the internet and downloads the next WU. crunching is referred to S@H, Seti @ Home, which is a project searching for extraterrestrial life and Folding is a project which simulates the way that proteins in human body change and mutate (the process is called folding, hence the name), its believed that when a protein mis-folds in our body, that can lead to serious diseases like Cancer, Alzheimer's disease and Parkinson's. the idea of this is try and find out how exactly do these processes occur, and what exactly goes wrong and how to fix this, and find cures for all these currently pretty much untreatable diseases
I think the popular belief these days is that the temperature fluctuations that a program like this causes is unhealthy for the cpu. Keeping your cpu pegged at 100% is the easiest way to ensure it does not change temps.
I think either way any cpu will outlive its usefulness before it dies. I am sure if I fired up the old hunk of junk 486 that I have sitting in a closet that the cpu will still work but would I really want to use a 33mhz processor?
I used CPU Idle for a while and saw much lower temps when my cpu wasn't in use. However, I also began having instability problems that I attributed to it. I've since re-installed Windows and don't see the need to use it again.