I'm a Google+ newbie and haven't tried this, but it seems to me that post-publish editing could completely change the meaning of the comment thread. I could change what I said and then the comments that follow would be responding incorrectly to a different thought. I could say, "I hate brussels sprouts." Subsequent comments could address that sentiment. If I change "brussels sprouts" to "lima beans," it would look like everyone is talking about lima beans, when they were actually talking about brussels sprouts.