Google tries its best to provide sensible default applications that cater to a generic populace but, as always, there will always be other apps that offer more or better features. In Android 4.4 KitKat, Google will be changing the rules a bit for third-party SMS apps that will, hopefully, make things easier for both developers and users.
From application launchers to lock screens to messaging apps, Android has made it somewhat easy to make even its core components replaceable and cutomizable. But SMS apps have not been so blessed, and apparently the situation is even worse under the hood. According to Google, developers of alternative SMS apps have so far relied on hidden APIs (Application Programming Interfaces) to build their software, a practice that is highly discouraged considering how these APIs can change without warning, tantamount to pulling the rug from under the developers', and users', feet. And so Google as decided to make those APIs public, which, in turn, will change how developers write their SMS apps.
The nitty gritty of the API changes can be found in Google's blog post, but in a nutshell, the new changes revolve around a very important new feature: the ability to set a default SMS app. Starting Android 4.4, only one SMS app will be able to actually receive message content, both SMS and MMS. All other SMS apps installed on the system will, instead, simply get a notification that the system received an SMS. Conversely, only the default SMS app will be able to write to the SMS provider, meaning only one app can actually send messages. On the user end of things, these changes mean that users will now be able to select their preferred SMS app from the Android system settings. When trying to send a message from a non-default app, they can be asked whether to switch the default messaging app, depending on how the app is written, or, in this case, rewritten.
Current SMS apps as they are won't crash in Android 4.4, but they might simply silently fail in some cases, which is why Google recommends that developers update their apps even before Android 4.4 is released. Which, if recent tips are to be believed, gives them just a few hours, or a day at most.