I detest anyone who would try to sap my spare computing resources without first informing me. With consent and full disclosure, however, I would likely participate in such a system.
So adopt the maxim: People who steal my computing resources aren't entitled to correct results. :)
Of course, I'm sure there are many others who would see nothing wrong with using a few milliseconds of the unsuspecting masses' time. I'm just not one of them.
Most people are perfectly ok with advertisements on web pages. I would guess that it wouldn't sap your resources more than downloading an advertisement or two.
In fact, for the average user, it might be less noticeable, given that you can load the page first and start loading the code/data for genetic programming afterwards.