Internally, G+ is marketed as a unified login/account system. Management's stated reasoning goes something like: * It's silly to force users to have separate accounts for Google services because most users would prefer to have Gmail, Docs/Drive, YouTube, Calendar, and so on under the same account/identity. * Users who want to have separate public identities will create pseudonymous "Pages" for each of their identitie…
I don't understand how this wasn't the case before Google+. I've had my Google account for years and it always worked on all the Google services I've seen. I've never had to use a separate account for anything.