Google embraces the proprietary, jettisons openness.
What was all that noise I recall people yammering about how Google is so much more open than Apple?
Google's messaging app is now, much like Microsoft’s offerings, a more proprietary, closed technology that is not compatible with other platforms. People who use open-source messaging clients like Adium and Pidgin will no longer have access to contacts who have transitioned to Hangouts. So privacy-conscious users will have no choice but to use Google credentials if they wish to chat with someone who is on Hangouts—or they’ll be left out of the conversation altogether. [From Google's Gchat Replacement Doesn’t Play With Other Instant Messaging Platforms]
Messages from Apple will be more open that Google's Hangouts.