Originally Posted by
phander Can a dev comment?
A bit of an old post, but I can comment.
Google Talk is just an implementation of the Jabber protocol. this is an open standard that anyone can implement. They had no user-base and so they needed inter-operability with other chat clients (this was at a time when Yahoo Messenger, MSN and AIM were the big boys.) And gmail was pretty new at the time as well.
So all these pointed toward using an open standard, and they did.
Fast forward to now, GTalk is a widely used IM system and they have a captive audience now. They wanted to guard their userbase by creating a "walled garden" of chat (which, to be fair, is just good business IMHO.) So they invented a new protocol that is specific to Google, and don't provide any public API, so that only approved applications can access it.
It may be possible to reverse-engineer Hangouts just as some have managed to reverse-engineer Chromecast (another "walled garden" which at least has an SDK) but in the end, they can change the API and break compatibility, because it's private and there's no contract to keep support for older versions.
The reason IM + continues to work is that google doesn't want to alienate their old users who prefer google talk or use alternative clients, so the Jabber interface is still active. Eventually they will deem it a necessary expenditure of users and purge the Jabber support so they can more tightly control their chat clients.
People talk about android and Google like they're all about the open-source, but when it comes to Google products, they are anything but. Android is just free / open source for OEMs because it increases the market penetration of proprietary google apps that are a revenue stream for them.