Do you work for a company that gives bonuses for filing patents? Do you use the "only for defensive purposes" assurances as a way to soothe your troubled soul when you take the money? If so, pay attention to this times two.
Ideally, we would just reform patent law to mitigate this but outside of that, I think it makes sense for companies to incentivize their workers to file patents "defensively".
I basically guarantee that that statement carries no weight at all.
They have a very valuable patent portfolio, and there is a very high likelihood that Google will at some point experience problems similar to Yahoo, Microsoft, Kodak, etc.
Once that happens, shareholders will be clamoring for that asset to be put to use - if management hasn't already done so to prop earnings up.
And that's why it's so pervasive - the only practical defense to patent lawsuits is building up your own portfolio of patents for defensive purposes. But, since it is highly likely that your company will face situations that require you to utilize that portfolio in an offensive way, your previously defensive patents will require other companies to acquire their own 'defensive' patents. Ad infinitum.
Of course, they're not doing that for everything (e.g. PageRank patents), but to the extent that they do this for the patents they intend to use defensively, they might avoid this problem.
I think James Gosling has a blog post about the huge number of defensive patents they started filing at Sun after losing some lawsuits over low-quality patents to make sure they would be protected in the future. Now all of those patents are owned by Oracle.
Of course, if Sun had been more successful, those patents might still be a net positive in the market, so it really isn't that simple. I do agree that the market incentives for companies are perverse (in terms of the constitutional purpose of patents), which is exactly why software patents desperately need reform.
Incidentally, Red Hat is a good example of an actively anti-software-patent company with a number of patents. It's unlikely that their policies will change, but it is possible. I don't, however, think it's a net negative when Red Hat employees file for them in the system we are unfortunately stuck with.