1) I had horrible experience with seti 3-4 years back. (system corruption) Not to mention I still haven't seen alien yet (or heard of).
2) Prime 95 sounds interesting but I lost interest in mathematics... ever since kindergarden. (Hah! And I am a developer and project manager
3) Unlike above 2, folding is not as agressive. It won't slow you down while you are web browsing, watching DVD, etc (granted you have over 2Ghz of CPU). In fact, its very good at handing back the resources to whatever demands it. I can run 3dmark and maybe see 2-4 percent difference if I left folding on.
4) This actually affects us in beneficial (or controversial depending on how data will be used) manner in relatively short time. (we are talking maybe few years down the road to have major breakthrough in medicine with our help)
5) Its great to test how system stability is after building one. (Even better than prime 95... I run 3dmark & games on with folding on to really stress my new builds of machines... it gets temp higher than any single method as it will stress both GPU & CPU near to its limit).