It depends on the reason for the back and forth. Is it to exchange new information and deepen the conversation or is it to retread the same ground and try to defend a point? When it's the latter it's rarely productive. If you were unclear the first time and someone misinterpreted you then perhaps take the time to make sure to be clearer in restating what you meant, but try not to go beyond that.

I remember going back and forth with you about that escher illusion a guy built in his garage. Just an example of what I'm talking about and why I think it's cool. http://news.ycombinator.com/item?id=2233584

About 5-10% of back and forth threads are useful to read. As you point out, these are usually civil debates about the merits of particular approaches.

I think the point is: as soon as it gets personal, get back to work.

The best predictor of a long exchange being good is the number of intermediate questions asked and answered in it.