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Applications with web-based front-ends have a fair share of advantages over native ones, even if you don't use them to collaborate with others through the Internet, and even if you run them on your own machine(s). Notes (including task / todo / buy-list management), personal finance management etc. would be good candidates, as it is useful to be able to access to these from tablets and/or phones besides your desktop/laptop, and having a web-based interface is still easier than having entirely different interfaces for each device type. (You will need somewhat different designs for different screen sizes, but it is still easier to use and develop than having completely different front-ends for each). They are also inherently multi-platform (for the client-side at least, but most web-apps are also developed in a portable manner anyway). Some of these services also happen to be managing information that one may not want to be hosted at a "cloud provider", so the ability to self-host is also important.
Of course, there are still many cases where a native application is better, but I expect that there will be more and more web-based ones coming. Edited 2011-11-06 10:11 UTC
Consider that GTK3 has a web based rendering back end available. It's entirely possible that in the future web based apps will be able to use the native widgets on whatever platform they choose.