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I agree that when something changes, you can cache the page and avoid rebuilding the page in the future, unless the page is changed.
However, sometimes it is intended that the caching should not rebuild the page even if it is changed - such as the front page here on xoops.org. It is not intended that everytime someone posts in the forums, the front page is updated - on the contrary, it is meant to be updated only every xx minutes (don't know the setting, but I'm guessing a minute)
I can see the potential and it is an interesting way to go - but don't forget that it CAN be of interest only to update the page once in a while and not on every page change.
