Earlier quoted context omitted.
You need to learn how defensive patents work. http://en.wikipedia.org/wiki/Defensive_patent_aggregation It's not just to stop from being sued over that particular patent, but to be able to use that patent to counter-sue when litigated for other patents. Google has yet to use any of its patents offensively.
Google didn't argue that this patent purchase was for defensive reasons. They argued that it was for offensive reasons, that these patents could then be used to attack Google and force licensing fees onto Android. That entire argument is negated by the fact that Google had the option of bidding jointly on the patents and, therefore, having a license to them.
Google doesn't just want to counter all Microsoft's future patents that they will buy, because that still doesn't change anything. They want to counter Microsoft's old patents, the ones they are using right now.