Martijn
I think the reason you will see web apps being used is that people end up needing an always-online system that keeps check of feeds when their own machines are offline. When you have the server based system, it often makes sense to turn that into a web app.
Martijn
That might still have you miss content from high throughput sites. E.g. sites who limit their feeds to the 5 latest items, but have made 6 posts between your computer's download moments.