And even if you do have a Google account, you might not want to log into YouTube with it; for instance, I recall reading that logging into YouTube with your Goo

And even if you do have a Google account, you might not want to log into YouTube with it; for instance, I recall reading that logging into YouTube with your Google account required you to put your real name into it, and if for some reason Google decided that your real name isn't your real name, you could lose access to your whole Google account, including more important services like Gmail and Google Talk.