Text messaging apps stand to get a whole lot better — and, yes, likely will include Google's own Hangouts app
The Android Developers blog wants the people developing third-party SMS apps to get ready for some big changes to come with KitKat.
The short version is that you can now "officially" make an application the default for sending and receiving SMS and MMS messages, as opposed to the old way of using hidden (not public) APIs and intents. That sort of coding works, but it's subject to change at any time and break all the apps that use it. The new method allows the system to receive and send messages, then use system defaults to decide where to display it. Developers should be sure to give the blog post a read, as the concept and new requirements are laid out nicely.
Now, what does that mean for us? Everyone wants the Hangouts app to send and receive SMS messages. The first step needed for that to happen is to make receiving a message (and sending one) a system function that can be handled by any default application. That's what Google is doing here. With this new method and API set in place, your Android phone doesn't need to ship with a "dedicated" SMS app, and you're free to download one that does things the way you like — or use the bundled Hangouts app.
Of course, there are other reasons they may be doing this. Getting rid of a dedicated Google Voice application (that is in sore need of a revamp) is one of them. Or maybe they all really like using Handcent. We should know more very soon.
Source: Android Developers Blog