Yea, this is an irritating trend, not confined to Google. It's getting harder and harder to have separate accounts and separate identities (or, god forbid, no identity) across services anymore. BIG BUTTON: "LOG IN WITH FACEBOOK!!!" Small, gray 6pt text: "Create an account with us."
How about no? How about neither?
I just want to browse the web. I don't want a relationship with you, company. And I really, REALLY don't want such a close relationship where you to have this unique identifier of me that allows you to correlate my activity on your site with my unrelated or related activity on some other site. It's none of your business. I don't need you aggregating everything I do, and I don't care that you're only doing it to "improve my experience" in some vague way. My web experience was fine and dandy before you tried scanning my rectum daily.
There are a scant few sites with whom I deliberately choose to share my identity, such as HN. This should be the exception.