Re: Unified data view?
It would depend on the architecture of the app. If the messages are stored on a remote server, then any instance of the app, regardless of its client environment, should be able to access the messages. If it's a game that stores a set of preferences locally, then opening that same game on a diffent device wouldn't access the same preference file, unless it has been synced through a cloud service.
The selling point for end users is access to more potential apps. But the real value here is for developers. If you can develop a Chrome App and have it run on mutliple platforms and devices, that's much less work than doing specialized builds for each device.