How often does one refer to somebody by URL? You may have noticed (I have, anyway) that FB now "helpfully" defaults to an @-mention if you start typing a person's name, essentially offering it as an auto-complete option.

As long as G+ doesn't go down this path, I can live with it, but I don't like that a "mention" automatically implies "full access" to whatever has been said, especially given that the choice of whether or not to share a particular post with that person is presumed to be up to the author of the post, not anyone else who was also included.

Agreed, The surprise birthday party is one exact case where this would be annoying, in almost every other situation most users would prefer the current behavior (In my experience anyways)