|The Google logo.|
So why the data sharing, you ask after adjusting your top hat and monocle? Google wants to ensure the best end-user experience possible, and to do that it must get to know you a little bit. Their go-to example is a search for "Jaguar". Do you want to know about the car, or the cat? Google can better guess what you want to know based on your past searches, or perhaps what news feeds you've subscribed to using News or Reader (other Google services). Frankly, I feel secure with my data in the hands of Google. They have a long history of refusing to hand over user data to law enforcement agencies, so I highly doubt they make a habit of handing it out to anyone.
My recent experience that put this in perspective was my acquisition of the Listen application for my phone. I have been doing a lot of commuting lately, and have been listening to podcasts to stave off boredom. Google Listen keeps a list of my podcast subscriptions and will download new episodes directly to my phone, and keep only as many as I wish. In order to easily facilitate this, it may be managed via Google Reader. Reader, for those who do not know, is an RSS feed manager. Put simply, it allows you to read the headlines/new developments from your favourite websites. If you want to read the article, you click the headline. If you do not, you scroll past it, and all is forgotten. Since I already use Reader on my browser (Chrome, of course), and my phone, I was able to seamlessly integrate Listen into my web-based experience. It was fantastic, it was easy and all because Google shares data between its services.
To close, I will say that you really shouldn't worry about Google sharing data between services. However, if you do, Google is notoriously transparent and allows you to manage your information via your dashboard. Also keep in mind that you probably use Facebook, and they definitely do not allow you any control over your personal information. So go ahead and Google. Also use Google+.