You can retain a subset of the functionality of the webapp while offline, but that doesn't really cut it all the time.
* Google maps won't work very well offline. You don't have the entirety of the map titles cached locally or the direction database/logic.
* It's the same with Gmail. IIRC, you don't have access to your archive of email while offline.
* If I have all my financial data in Mint, do I have access to the full functionality of Mint or just access to my account info / transaction history? If not then it's sort of worthless.
* If I upload all of my photos directly from my camera to Flickr or Facebook, do I have access to those offline?
I'm not poo-poo'ing the idea of webapps, I'm pointing out their weaknesses because it seems that most people are too focused on their strengths while ignoring their weaknesses.
My other point is this:
As we more more and more of our data onto servers controlled by other people, what are we giving up? Sure we can keep backups of the data just in case the remote data is lost (for whatever reason), but what happens when the only means of using that data is a webapp? Then the backup becomes useless without the service. (i.e. I could be offline and have all my financial data in some sort of backup format from Mint, but how do I grok the raw data without a local tool? If all the tools are online, then it's useless (don't talk about offline webapps here, because they can't access/parse local data IIRC)?