My conspiracy theory is that this app will eventually replace the Hangouts app.

Notes:

1) This is not AOSP. It is closed source. And since it can be updated through Play, Google can ship new features independent of Android releases. Some say good (I do). Some might label it as a trojan horse... for Hangouts.

2) For Lollipop, carriers choose between the classic AOSP Messaging app and this one, Messenger.

3) Hangouts suspiciously has not received the Material Design treatment.

4) Google is holding Material Design hostage to get Messenger onto the phone as the default SMS/MMS app, possibly to deliver an even more integrated Hangouts later.

5) Wonder if Messenger is part of the Google app preload required to get the Play Store on the device?