For a concrete example of the power of Web Intents, consider this: On Android, a browser (whether the stock or an alternative like Dolphin or Firefox) could (and probably will given the architecture / history) offer some local Android Intent handlers as handlers for some Web Intents.

To take the example from the blog post, at the photo storage website you could choose to edit an image and get menu of choices that doesn't just include web app image editors, but also includes native image editors installed on your device. That sounds pretty amazing to me.