It's persuasive until you actually try and do it. Then you find that you can't make anything nearly as nice and it takes you 10 times as long to do it because you spend forever figuring out weird quirks of mobile browsers that make not much sense at all.
Or you use the web technologies for a "native" app and focus only on the browser for that platform. Then port to others as needed, rather than trying to support them "all" from day one on a web app.
Certainly a web app is usually still not as fast and responsive as a native app, but its getting much better.
People used to say the same thing about web mail services - that they were not nearly as 'nice' as a native mail app. Now go look at how many people use Gmail and dont bother with a native mail client on their PC anymore.
Consider how many iPhone apps are just "website silos", an app with an embedded UIWebView hosting a (live or static) website that looks like a native app.
Also, the WebOS SDK allows developers to write native code plugins that can be called from JavaScript. So an app's high-level UI logic can be written by web developers productive with JavaScript and C++ developers can port legacy C++ code or optimize performance bottlenecks with WebOS plugins.
