1. Although slower than using your own GPU(s), this scales with your visitors. Your own hardware does not do that.
2. Most visitors don't spend a long time on your website. If you efficiently distribute work amongst workers, you can probably even leverage the shortest of visits.
3. I built something like this called MapRejuice (CPU instead of eyeballs) for a hackathon once. It ran MapReduce jobs pretty quickly (we embedded it into a relatively popular Twitter widget ... heh). One of the biggest obstacles to commercial application was security (not a lack of processing power though!).
If bitcoin takes off, people will just take the "free money" their machine is creating for themselves. I'll be the first to make a browser plugin that mines bitcoin miners :)
In any case, great effort. I definitely think there is merit to the idea of using all the idle processing power in the world instead of building huge new datacenters. Bitcoin is probably not the best application for it though (research stuff like DNA folding seems more appropriate)