Google Chairman Eric Schmitz came out and admitted that Google bought Motorola for its amazing hardware. Given how people believed Moto's patents, while considerable in numbers, are not enough to protect Android, this seems to make more sense.
It isn't as if Google can buy HTC. Google is rich but they cannot afford Sony or Samsung. So it makes sense for them to buy an Android device maker more local and probably more familiar with. By local, I mean in the US without cultural and language barriers.
It's anyone's guess what Google's plans are but a good bet is that there will be tighter OS and hardware integration along the line of what Apple has with its iOS devices.
It's a risky move but one that could pay dividends if it works out as planned.