Due to Google's reliance on Linux, it isn't a surprise that they chose to join. As they mention themselves, if you use Google you are using Linux:
You'll often hear members of our open source team say, “Every time you use Google, you’re using Linux.” It’s absolutely true. Check a Google engineer’s workstation, and you’ll probably find it's running Linux. Do a search on Google.com, and a Linux server will return your results. Ever since Google got its start, Linux has given us the power and flexibility we need to serve millions of users around the world.
Now, whether or not the patents and technologies they bring will help foster development in the community as a whole is a different story, since not all patents are inherently useful to developers. You can read the full blog entry on the Google blog.