If you use Google Patents (Beta) , you can search patents and that allows you to find patents around a technology and who else is citing a patent. So let us say Apple cites a Motorola patent and Google acquires all of Motorola's patents its hard for Apple to prove that Google Android or Motorola devices infringed their intellectual property (IP). "Hey – you took our IP because you refer to the Motorola patent in the first place…" is the sort of neat answer that Google can give.
While analysts maintain that Google wants to get into the cell phone hardware business- I think the patent defense idea is probably the bigger reason for the Motorala acquisition. Google's credo is "do no evil" but with Apple ,Microsoft,Oracle constantly alleging infringement – a rather evil motive, the Motorola patent buy is like getting nuclear arms by a country – serves as a real deterrent to attacks.
And the fun part is that Google Patents allows anyone to search patents instantly and Apple and Microsoft could have come to the same conclusion – viz . buy Motorola's patents. You can be sure that many industries like green energy,pharmaceuticals,electronics will be trying the same route of acquiring a whole bunch of patents to serve as a deterrent to competitors infringement allegations.