If the purpose of a 'wall' is to have something that allows you to read something later, the stream already does that. You can go to your notifications, and read whatever someone has specifically shared with you, or tagged you in. You can do that when you get the notification, or even go to your notifications stream to read it later. In my circle of friends, a wall also serves to allow others to see what has been shared with you. +Christina Trapolino
, here I agree with you, I have no interest in having people see everything that has been shared with me. It detracts from the usefulness of circles. For instance, I would never add my mother to Facebook, because I don't want her to know everything that happens in my life, like a great party over the weekend. However, I will gladly add her on Google+ because I have the ability to limit what I share with her.
I'm sure when the search feature arrives, I will be able to search for posts where I, or my friends were tagged in, or that they posted. If it was shared privately with me, then I can just browse my notifications.