I believe this is there goal for Q1 2014. Which will be awesome for Sprint users using the Google Voice integration. Here's how it works now (for me, with Sprint and GV integration):
-SMS texts to and from one other party come in on GV app on android, iPhone, iPad, laptop, and desktop chrome extensions, all at same time (it's effing sweet). This is done over data
-hangouts messages to my other gmail brethren who have it installed work flawlessly (and better than the older SMS standard).
-MMS messages - only come in via the stock android messaging app. iMessage group chats with all my iPhone friends come into that app, and I must use that app to communicate. This is over the completely sh*tty MMS standard, and cannot go over data (read: wifi) unless your carrier supports that.
Here's how I envision it working after they make the updates to GV to allow MMS and SMS messaging with your GV account:
-all SMS messages will come into hangouts apps on: android, iPhone, iPad, laptop, desktop, chrome extensions, etc...
-all hangouts messages will obviously do the same, as they already have in the past
-all iPhone group chats sent via iMessage, which uses MMS as the standard if there's non iPhone's in the group, will now come into this same central Hangouts hub, and be accessible on all my devices, at the same time, over data.
What does this mean? WiFi on the plane? I won't miss a beat of what my friends are up to when they create group chats. Obviously if everyone adapted an open standard aside from MMS that didn't suck, this wouldn't be an issue. But that isn't happening anytime soon (although an interesting read for those interested:
You have too many chat apps. Can Layer connect them? | The Verge)
Hope this helps to answer your question... that's at least how I envision it working after google finishes all the necessary changes.