There are those who will find it useful but others will find that there are better desktop tools for these purposes. Individual developers are free to pick and choose.
Fortunately, neither this announcement not the Ipython roadmap emphasises development of the web-based notebook.
These already exist, though, and the ecosystem already has a lot of support and momentum behind it: javascript within the browser.
Would you explain what some of the benefits are in comparison to this? I'm an outsider so I don't have a very firm understanding of what the implications are.
You could certainly base something like that around Javascript, and it would have certain advantages. But the Python world has a lot invested in scientific computing - Numpy, Scipy, and dozens of packages built on top of them. It's also got good integration with other languages, from FORTRAN to R. This sort of scientific computing is where we think the notebook really shines.
There is a development branch of iPython by Brian Granger that emits JSON when you display a python object in the notebook. You can easily create handlers that display any Python object using javascript. That means D3.js or whatever HTML5/WebGL app you can throw in there becomes the front-end to a very powerful numerical computation back-end. Check out this demonstration I help code, http://www.flickr.com/photos/47156828@N06/8183294725