For Microsoft, XMPP federation is a great way to contain Google and make inroads on places that are Cisco IM/phone shops. In the long run, they want to extend corporate email dominance to phone via Lync/Skype.
Google doesn't want to be a commodity. Hangouts is their secret sauce. What do they gain by being open?
The support of many geeks worldwide (and subsequently their friends and customers), just as it happened until recently when their "do no evil" transformed in "screw you". Google (gmail, gtalk etc) would have never risen to this level of popularity without them.