Sure Google can computer more for less, but I already have a PC; why buy two (one for Google and one for me)? SETI@Home leverages... (wait for it)... the desktop!
Second, the point here is not about utility computing. It's about developing applications. Let's face it, web apps are struggling to the the things that most desktop apps do easily. (Ajax is a joke.) Certainly they both have their pros and cons. And there are certainly some situations where a web app makes more sense. Don't get me wrong. But when creating rich applications, the web sucks. And all your arguing here won't change that.