There is a very good article by Gabor Cselle about what's missing in Web 2.0. One of the things he draws out is the inability to take these Web 2.0 applications offline. While someday we'll see the world covered with internet access, right now, this is a very big deal for both business users and home users. It's also where Rich Internet technologies like Flash and WPF are going to shine and really differentiate themselves from other web solutions.
The power of these technologies lies in their ability to operate outside of the web browser. The plugin model has been derided by some, but this provides a direct link with the operating system. The inherent nature of Flash and WPF is such that you can run applications regardless of an internet connection. Running a .SWF on your computer works regardless of whether or not you have connectivity (though functionality may be limited).
The piece of this puzzle that has yet to fall into place is being able to synchronize data from the internet and store it for later use. Adobe's Apollo project aims to do exactly this by creating a desktop runtime that can seamlessly take data offline. WPF's very close link to the operating system should also provide the ability for WPF/E applications to store data for later use (perhaps backpacking in Wyoming's Wind River Mountains).
Once this link comes to fruition, the lifespan of desktop applications that connect to the internet is going to be cut down severely. Why use a client like Outlook when you can have the same functionality (and more portability) from a Rich Internet Application? The ubiquity behind the technologies, as I've mentioned before, opens up the possibility of taking your offline content anywhere - to your phone or your media center.
The real elegance of RIAs is that they will behave like desktop applications regardless of whether you are online or offline. When connected, which should be usually, they will pull content and data from all over the web. When offline, they will allow you to manipulate and view that content. When you go back online, those changes will be updated. And this is all in a package that goes anywhere and feels the same no matter what OS, device, or computer you're using. That ubiquity and the consistent experience is what is going to draw users.