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I think you hit one nail on the head when you mentioned patents. Motorola has 17,000+ patents. As we have seen over the past couple of years, patent litigation has become an industry into itself. Google knows a little about this, with that little ongoing unpleasantness with Oracle. It can literally be cheaper to buy the company that holds the patents instead of risking litigation with a nearly limitless potential for loss.
But if you want to know the other half of the reason Google purchased Motorola, just glance around your office at the iPads. Now look for an Android tablet. Cue crickets chirping. Google is not a stupid company, and they are painfully aware that Apple is milking the iPad gravy train all the way to the bank, while all of Google's mobile eggs are in one smartphone basket (Jut try to mix more metaphors than that - I dare you.). Google is going into the tablet business, my friends, and if I was Apple, I would expect some serious competition in the tablet market in a year or so as a result.
The acquisition will enable Google to supercharge the Android ecosystem and will enhance competition in mobile computing. Motorola Mobility will remain a licensee of Android and Android will remain open. Google will run Motorola Mobility as a separate business.