Uh... why? Because it's the new buzz word?
Cloud computing is a great idea, and it opens up a wide range of possibilities to the world of parallel programming. Since we seam to be nearing a plateau in how good we can make a single processor, that basically means it opens up a wide range of possibilities to the only way (currently) we can expand our computing prowess in a meaningful way.
As far as data analysis goes [e.g. SETI@Home or Folding@Home projects], I agree that cloud computing is an effective means to accomplish your goal. But cloud computing, as far as a home-user daily OS is an awful idea. From what I understand about it, which probably isn't as much as you, the Cloud takes everything off of your local machine to be stored on an external machine. Other than obvious security/privacy issues (I may not be as big a zealot as iago is on security, but even I've got my limits...) it also doesn't give me the ability to set up my machine the way I want it. There's more but I've got a physics midterm to study for/fail.
Modern computer performance is entirely sufficient to support the kinds of tasks most people use their PC for.
Of course the only places where Cloud Computing is objectively advantageous is in computationally complex areas like the ones you named.
You made a general statement. Cloud Computing is by no means a stupid idea. Of course, it's not exactly novel, but discrediting any sort of distributed computing is completely silly.
There are a variety of applications (especially in academia) where security of relevant data is a non-issue.
