On the very face of it, Google doesn't need a handset subsidiary to make a custom Google phone. They can easily commission a handset exactly how they want it from OEM brands like HTC and Samsung, and have already done just that with the Nexus models. They could easily design their own brand of phones and have it built by contract manufacturers like Foxconn (as Apple does). I've read elsewhere that Google now gets the benefit of better understanding of the challenges of integrating Android into handsets. That's also incorrect. Google has a history of sending teams of engineers to most of their handset and tablet partners to work side by side overcoming those challenges.
Perhaps Google just saw a good deal on Motorola. Its stock price has been dropping through the decade, and also the past year. The market value prior to today was just $7.3 Billion, compared to $20B for Nokia or $13B for RIM. Perhaps, like Nortel before it, the value of Motorola's Intellectual Property (IP) is being hidden by a poor operational record.