Click here for the first part of this answer.
Rich clients are great. They are fast, they are beautiful and they do just what you want in an elegant fashion. They will be very important in many scenarios (the intermittent, crappy, GPRS connectivity environment of cell phone data applications comes immediately to mind). However, an infrastructure like Web Forms begins to close one of the end-to-end doors. You no longer have the same control over significant bits of the client and server ends of your architecture. While the tools allow you to work faster and generate more stuff, one of your cyber-civil-liberties gets constrained. If you don't like bits of the WebForms infrastructure or if you want to innovate/renovate parts of it, you are not allowed to do so by the vendor.
So endeth the soap-box and philosophizing. Here is what I think.
1) Both modes will coexist (yes the convoluted way is here to stay)
2) All the standard arguments continue to be valid promoting "dumb" Web protocol-based development including portability and end user reach.
3) Rich clients with dumb data (e.g., just XML, no prescribed HTML formatting) is a great architectural solution since you expand even further the possibilities for the client to interpret your data and to render/syndicate/use it appropriately.
4) WebForms is a framework that implements the architectural theme in #3 within the context of a specific server and client framework.
5) To the extent that WebForms makes implementing a good architectural pattern easy, I think its great. To the extent that it locks you into a specific framework, including schema and interactions that will not interoperate (easily) with other frameworks, I think you lose many of the benefits of the architectural approach.
FWIW, I'm not recommending or discouraging either approach as no single approach fits all implementation scenarios. WebForms is a great new addition to our development toolkit. As with any power tool, however, you need to understand how and when to use it unless you happen to enjoy the hustle and bustle of the Emergency Room.