Your justification for tying everything together, not only a single method of signing on but a single pervasive identity, was just that ... it's a single person

Your justification for tying everything together, not only a single method of signing on but a single pervasive identity, was just that ... it's a single person using the things. That's not a very good justification. And your example of a Good Thing that comes from it, by way of contesting my claim that youtube and gmail (or g+) are conceptually distinct, used something else entirely, involving email and planes. (And actually I would prefer those things not to be done automatically!)