I do not see how the + sign has anything to do with Google+. :)
It is just a matter of balancing the importantance/"commoness" of actions.
By each thing you make directly visible, everything else becomes a little less visible. Google simply pushes the simplicity factor to the edge in their visuals, like always.
For example, it might be faster for some to find the smiley if it was not hidden behind the +. But it would take longer for others to find the more important/common attach button, if all other buttons, like the Smiley button, would always show.
You could ask the same question about a lot of things. For example the Formatting Options A button, or the More button in the top bar of google pages.
I find it successful. This balance is what makes it possible for my 83 year old grandmother to be able to find the attach button.