One of the major complaints with the Hangouts app on Android is that it doesn’t integrate with the other built-in messaging apps on Android. This leaves you with not one but three messaging apps when you first start the device, Hangouts, Google+ Messenger and the standard Messages app for SMS and MMS.
While the jury is still out on why Google needs both Hangouts and Google+ Messenger app since they both essentially do the same, a lot of people have suggested Google integrate Messages with Hangouts (the way SMS, MMS and iMessages are integrated in a single app on iOS or Facebook chat and SMS are integrated on Windows Phone).
It seems this wish will be granted soon. According to Android Police, Google is working on integrating the SMS and MMS functionality within the Hangouts application in the upcoming version 1.3 release. The messages sent and received over SMS will have a different stamp next to them so you know what protocol was used for them. Same goes for MMS messages.
Android Police speculates that the Hangouts app will completely replace Messages app on Nexus devices starting with KitKat. Whether the OEMs will follow suit is anybody’s guess.