Red,
Any program that uses the CPU in any way, slowly kills the CPU. RC5 however, forces the CPU to run at 100% load 24/7, thus it "eats away" at the CPU many times more quickly than any other program would (other programs like RC5 kill it too. Seti if its running in the background, etc)
To many people, the CPUs life will outlast RC5 because the user upgrades his/her CPU fairly often. To others who do not replace parts as often, the CPU may die before the user intended on upgrading.
To be totally honest, a while ago the posts asking people to join RC5 were not too bad, and were few and far between, but now there are many many more threads about joining RC5.
BTW Red... Look at my nick. Do you see a PH in it? its STEFAN!! Someone has eventually got to notice that!