Starting with Android 4.4, Google wants to change that and make it so only one SMS app can receive the SMS_DELIVER_ACTION intent or the WAP_PUSH_DELIVER_ACTION intent for MMS messages. Also, if an app is not the default SMS app on a device, Google is asking developers to disable the option for that app to send a message. It seems that although there are quite a few SMS apps in Google Play that will handle your messaging for you, most are using hidden APIs to do this. Google says it "discourages" this practice, but we haven't heard of the company ever banning an app from Google Play for doing it.
So, with Android 4.4 Google is offering developers a standard API set for building an SMS app, and that's where this change will come in. It also means that if an app is using the hidden APIs, but doesn't update, it will not work with Android 4.4. Of course, at launch that will only mean a few unsupported devices, but those out there with Nexus devices tend to be a very vocal minority.
source: Android Developers Blog