Users who would like to be able to "flag" posts that they wish to find later, should all use the "Send Feedback" tool within Google+.
This has been something that users have been mentioning in the Help Forum since the beginning of G+. I would like to think that enough requests (through the "Send Feedback" tool where they can track the numbers) would prompt Google to do something about this. So, perhaps not enough people are using "Send Feedback?" Surely such a feature's absence is not because it is too difficult for them to implement.
In the meantime, for people who don't want to create a circle, add themselves to that circle, and share the post with that Circle ... they can open the individual post and add it to their browser bookmarks/favorites.