Tim Bray wrote a little blog post on Web vs. Native apps and makes a really important point: actually, all apps these days use the web. The only distinction is the execution environment.
I wrote a comment where I noted he missed a very important property that webapps mix well together. The same cannot be said for Android or iOS apps. Native apps can pull in resources just as well as webapps, but they don't do mashups. The data-structure of the web-interface, the DOM, is understood well enough that you can enable collaboration between programs.
After I posted this comment it occurred to me that this common format, the DOM, makes certain UI consistent across applications in a way that native apps don't achieve. You can select any text content, for example. You can zoom in and out of a webapp. You can send people links to parts of a webapp (maybe).