The executive summary is that third party messaging applications, like Textra or Handcent, can now be made the default app for sending and receiving SMS and MMS, as opposed to the way they currently do it by exploiting private / hidden API calls. This style of programming works — and works quite well — but it isn’t without its flaws. Prime amongst these is that apps using these calls can be broken at any time by changes to unpublished system calls, and then all of a sudden your preferred SMS app won’t be displaying your messages anymore.
If you’re not a developer, this probably doesn’t matter too much to you right now. However, it could be a sign of things to come. Google’s messaging app (or an OEM’s replacement messaging app) is a system app for the moment, and there’s no official means for a third party app to become your messaging app. With this change in Android 4.4, that will be possible. This means that the much wanted feature — Hangouts incorporating SMS and MMS to give us a unified messaging feature — will now be entirely possible using the proper channels.
With this continued decoupling of user interface from Android’s core functions, the system will handle SMS and MMS transmission, and allow the appropriate app (selected by the user) to display and generate that content. A default messaging app is bound to be provided, but more readily replaceable under these announced changes.
There’s no official word on whether Hangouts is going to include SMS or MMS at this time, or when Android 4.4 launches, but it does seem a good fit.