Well right off the mark, TheOpenSourcery.com got the principl reasons for the buyout – right on Denmark.

See here – 1)Google is going to operate the company as is and not bring Mototrola into the Google fold – this means minimal disruptions and has later advantages;
See here – 2)Google has got its major other hardware partners on board;
See here – 3)Google is reiterating its open pledge for Android – its chief selling point to hardware makers. Thus a major Linux influenced OS will remain an open player in smartphones and tablets. What the partners will find hard to digest is that Google will get to declare when the next version, as in Honeycomb, becomes “open and available” to partners. Yes, this is the Microsoft system of making various versions of Windows available to its OEM partners. Big difference – Google does not charge those partners for each Android unit sold[right now MSFT is taking a big chunk of the total price  in the PC marketplace]. Also Google has been reasonably timely with full and open SDK releases[compared to Windows SDK , Mac OS and IOS – much more complete disclosure let alone timely];
See here – 4)Google now has a hardware arm where it can get fast feedback on software+hardware ideas and inventions. A Nexus like foundry to set minimum standards for Android hardware. But again Google’s OEM Android partners could certainly have valid concerns.
See here – 5)Google gets some key mobile patent protections that it would have had to pay $6B+++ for – about 17,000+ many more in the process of approval. This is ostensibly the main reason that Google did the  deal. But there is plenty skepticism . See WSJ. See NYTimes. See Reuters.  See the Economist. And for best round table of ideas on buyout, see Engadget.

Final observation – patent mischief makers in the mobile realm[think AAPL and MSFT primarily] – Checkmate!

Ye Editor agrees with the Engadget team – this buyout is more than patent defensive; but is an enabler for Google in so many aspects – the company made a defensive move with huge offensive potential. “Do no evil” will get tested again; but compared to AAPL  of late and MSFT, ye Editor chooses Google for now as better and more ethical businessmen in the technology realm.