I, for one, think that web visitors should at least be told what's going on, even if just a little white lie ("We're testing your CPU for blah blah blah..."). If I visited a website and noticed my CPU spike, you can bet I'd close that window and never come back (and possibly report to various malware entities). Or at the very-very least, the home page should not have that script running, and only if they go further into your site. A proper homepage needs to load fast, get the information to your visitors, or you're not going to have many visitors.
Just try to think of the ethical side. Also like mentioned earlier in the thread, maybe for registered/paying customers, the script is disabled (or give them the option to disable it). Above all else, tell visitors what's going on, or you run the risk of being marked as malware and/or lawsuits.
^Everything should be above the board, people should be down with "Ad Free Internet" I think this idea gets confused with a "Free Internet"
I hope new coders see this thread and contribute to this type of mining