What I love about web apps is that they're so ephemeral. You open it, try it, close the window, and it's gone. And it only comes back if you ever go to that domain again
Downloading an app, worrying about spyware, having something run constantly in the background, no thanks
The Internet is for network protocols and documents.
Although I develop web applications, not by choice, I'll take a native desktop application always over a so called "web application", when possible.