Just because you can...

...doesn't mean you should.

It's an interesting concept, I'll grant them that. You /could/ say it's even easier than making a cross-platform offline application that syncs with online data (but that's debatable).

Unfortunately it completely negates the best features of web applications - such as their own Google Maps - which just can't work offline. They're re-inventing the wheel with this one, but perhaps making it slightly more round.