This was a nice overview of RIAs, and I appreciate your covering this topic....but I think you're kind of missing the point when you mention that a drawback of Flex is that search engines have trouble spidering it. No doubt, they do (that is really a defect of search engines). But does it matter?

Flex is designed for building rich *applications*, not websites. You may use it to create a widget for a website, but by and large Flex is for interactivity, not for content delivery. And given search engines' ability (or lack thereof) to interpret JavaScript, the issue does not apply only to Flex - no RIAs are particularly searchable (even Google admits it: http://tinyurl.com/2h9gtg). Flex does, however, have some fast-improving accessibility features.