Here are the issues that I have encountered with the problem:
Commenting on a thread (thread A) to let the people there know that you are having the conversation elsewhere (LINK to thread B), has a tendency to cause disruption. The commenters who are involved in thread A feel disrespected and tend to lash out at the commenter posting the link to thread B, which results in thread A being completely overwhelmed with comments about the propriety of posting a link in this manner. As a result, the conversation that was unfolding on thread A is lost. The ill will by some of the followers may manifest in their ignoring thread B completely and missing out on the conversation there, or retaliating, by posting annoying comments or bombing thread B with links to thread A.
Mixing groups of people can water down the conversation or effectively discourage commenters from participating. Say for example, someone who writes for a science network and has a science-heavy audience on Google+ and a columnist who has a varied audience on Google+ both post a link about an interesting scientific finding at around the same time. The conversation that develops in the thread among the people with a scientific background will be very specific, critiquing the methodology, asking more questions about future studies, reflecting on what the findings mean for other areas of research, etc., whereas the more varied audience may be more interested in how this applies to their personal lives and how this is something they suspected due to the amount of anecdotal evidence they have.
These two audiences create very different content, content that is valuable for different reasons. but which would suffer if we were to merge the threads prematurely. Anecdotal evidence is not something particularly embraced by the scientific community and might impede their desire to really get into details with people who clearly would not understand them, and the technical language used to discuss the finding would put off the varied audience who may not feel they have sufficient knowledge to express something intelligent about it.
Already in discussing this topic with people in my circles, people in +Alida Brandenburg
's circles and those of you who commented here, I have seen a lot of incredible variation in suggestions and points. This thread, for example, has substantially more critiques of Google+ features than the other two -- and I wonder whether these would have been pursued at such length had the comments occurred on my own thread. Probably not. The joy of having varied audiences and being able to tackle aspects of a discussion is what makes this network great. Perhaps the best solution is to do what +Ahmed Zeeshan
did here, wait for the discussions elsewhere to die down, and then bring the post to the attention of his audience to discuss further.
The matter of being able to find the original post from a reshare is of great importance, however.
If you are interested, +Alida Brandenburg
's post is here: https://plus.google.com/103765013042311928518/posts/ax1vDrezM34
And my original post is here: https://plus.google.com/112254467169147545881/posts/NqBEhyMy4j8
The comments on Alida's post are exceptional, if you click to exit this thread at all, go there.