The official Google Blog:
People have told us that accessing all of their Google stuff with one account makes life a whole lot easier. But we’ve also heard that it doesn’t make sense for your Google+ profile to be your identity in all the other Google products you use.
So in the coming months, a Google Account will be all you’ll need to share content, communicate with contacts, create a YouTube channel and more, all across Google.
“We tried to force you to use it. That didn’t work. Sorry about that. Sorry it didn’t work, we mean.”