As far as I can tell, the advantages of using a RIA (rich internet application) toolkit are:
* easier to develop desktop-like web applications
* more straightforward to avoid duplicating client-side and server-side logic
and the disadvantages are
* your page tends to load significantly slower
* your page isn't accessible to people browsing without javascript or disabled folks
* you'll still have to generate html for googlebot, etc.
Things I'm not sure about:
* difference in development time
* whether it's equally easy to integrate real-time functionality when you're using a RIA toolkit
What did I miss?
(other examples of RIA toolkits are http://www.sproutcore.com/ and http://cappuccino.org/. SproutCore is notable for supporting mobile devices unusually well, so I guess that might be a point in its favor?)