I suspect the sudden change to XMPP was because Google was going to lose ad revenue because of Microsoft adding support for Gtalk in Outlook.com.
Google is most effectient at showing relevant ads if they can track people across all their PCs and devices. Gmail is perhaps used as a "loss leader" to make people sign in into Google, so that cookies can be set and your search keywords and activities can be tracked across your home/work machines/laptop/desktop etc. No wonder the first page that Chrome shows is asing you to sign into Google.

Now, if people who use Gtalk to talk to their friends switch to Outlook.com but sign in to Gtalk there instead of into Gmail, Google cannot set a cookie for Google.com under your Google account, defeating the tracking needed to for proper monetization.

Solution? Hold users' social graph hostage and make it more difficult for them to switch by dropping XMPP support, the OSS clients be screwed. Instead of trying to lock-in people by holding their own data hostage in the GOOGOLCLOUD I wish they would just improve their offering compared to Outlook.com. The new compose in Gmail is atrocious while Outlook has a fresh new look.

Also, I have an unfounded suspicion that Andy Rubin might have been the "open" guy at Google and might have left or forced to leave a few months because all these changes don't come in a day(Andy was the guy behind the famous "open = build src" tweet, and now that Vic Gundotra has even more power, he's bound to do even more such things. See how Chromebooks are locked out, and flipping the developer switch is much more of a pain(flipping it erases the ROM and you need to press Ctrl-D or wait 30 seconds looking at scary warning every single time you boot in dev mode) than even turning off UEFI secure boot. They're closing the door behind them that they entered through, i.e through Chrome on Windows which is more "open" by comparison, making sure that Firefox and other browsers can't run on stock Chromebooks and keeping out all native apps except heavily sandboxed network apps.

Well, at least they are closed upfront about Chromebooks, instead of acting open and getting people's social graph into Gtalk and then locking it up in a proprietary protocol as hostage to artificially erect barriers prevent people switching away from Gmail. Atleast MSN, Yahoo, Skype were upfront they weren't really tolerant of third pary apps.

When Google killed reader and a lot of folks where whining about not trusting Google APIs, I thought there were just paranoid, but it looks like it's all slowly falling into place. Google isn't "open" anymore and they're coming really close to doing evil, if not already there.

As a side note, have you looked at Google VP's Vic Gundotra's profile? There might be a clue about where all these lock in tacting are coming from.

From http://en.wikipedia.org/wiki/Vic_Gundotra

Vivek "Vic" Gundotra[1] is the Senior Vice President, Engineering for Google[2] and was previously the Senior Vice-President of Social Business for the same company. Before joining Google, he was a general manager at Microsoft.[3] He joined Microsoft in 1991 and eventually became General Manager of Platform Evangelism. His duties included promoting Microsoft's APIs and platforms to independent developers and helping to develop a strategy for Windows Live online services to compete with Google's web-based software applications.[4] Gundotra joined Google in June 2007, after taking a one-year delay working on charitable endeavors[which?] due to a Microsoft employee non-compete agreement.

His responsibilities as Vice-President of Engineering include Google's mobile phone applications, and its developer efforts, including OpenSocial, Google Gadgets, Google Gears, and the more than 40 product APIs Google exposes. It was reported on Gigaom that he is considered to be Google's Social Czar. He is apparently known to be the man behind Google+,[5] and is responsible for the controversial removal of social features from Google Reader.[6]