[blader]

They are good for some type of applications that are similar to document based (e.g facebook, flickr, HN news), but for others, web apps are terrible (twitter, photo editing, making phone calls).

I'd like to know why you think this is true.

That observation may be true today, but it is getting less and less true every day. The evolution of technology infrastructure over the past decade has been in the direction of making web applications more and more powerful. (Javascript, Flash) The overarching trend points to web apps taking over the domain of desktop apps inch by inch.

Name any popular desktop app and I can more likely than not point to some company or other porting it to the browser.