I didn't know that the +1 button allowed to share, either. And I always found that very annoying.
I actually used the +1 button only one single time (probably mainly to try out the feature). For all the other articles on the web that I found interesting, I copied the URL, went to G+ and pasted it into the share box. Which is really tedious,
And I did this precisely for the reason +Michael Kelly
talked about: I don't want to just share something without adding any comment to it. When I share (or +1) stuff, I always make some kind of a statement, implicitly. The problem with implicit statements, however, is that they are completely open to interpretation. Did I share an article because I agree with it? Or because I disagree? Or because I just find it interesting without having an opinion about it yet? Nobody knows.
So I prefer to make explicit statements by telling people why I shared something and what I think of it.
So for me, only a share button works.