That's actually pretty good advice, except it may be hard to find legal and moral things to run. We wanted to find the cure for cancer and not produce rainbow tables or do DDoS attacks.
Well we noticed that flash ads sometimes take up even more than 100% of a CPU (meaning it can spawn threads and use multicore processing), video ads perhaps even more since they may get to be gpu accelerated, as CSS3 animations. We figured if people are spending this much CPU cycles for advertising, than why not clean up all the advertising and use the CPU cycles for some protein folding and finding a cure for cancer to make a website owners, visitors and a group of researchers happy ?