They've just bid a billion dollars for a patent portfolio, which they claim is a necessary defensive move. If you accept that, then applying for patents is a valid defensive strategy too.
Yes, if you define "defensive" to mean that they want to guard their proprietary forms of innovation against others who might seek to use them during the period of the temporary monopoly they hold on them. But not defensive in the sense of having been obtained in anticipation primarily of being used in litigation defense when others sue Google (which, I think, is how "defensive" is normally used on this topic).
I am no special authority on Google's thinking in this area. Its public actions and comments, though, suggest that it is a company that dislikes and even despises low-quality patents used primarily to foment litigation while supporting the types of patents (like PageRank) that reward innovation. That is pretty much how I read this piece. By the way, I also have an antipathy to flaky patents and strongly believe that the patent system needs reform, but I am not opposed to IP generally, nor to patents (when done properly) in particular.