A more important question is, "what do you think of a person getting back together with an ex lover" and my answer to that, for either a mono or poly person, is "Bad bad idea." I never understand why people think it will work any better the second time. People break up for a reason, and most second runs end the same way as the first ones. Sure "people change" and it is theoretically possible that given enough time, both people will change enough to become "a whole new person" but by the time they grow that much, their romantic interests tend to change as well.
I guess I get to be one of those exceptions as well. Cajun and I broke up for 10 months. I moved back to IL (400 miles away) and even started dating someone else (at the insistence of friends and family). The reason for our breakup was because he "could not see us being married."
We are going on 4 years this coming Thursday. We had both grown enough to know that we still wanted to be together. I had grown enough to take better care of myself. He had grown enough to accept that things don't always turn out the way he wants but they can still be wonderful.
I know you said that your idea goes for "most second runs" and that it is "theorectically possible to change" to become a whole new person, but sometimes (as in our case) no one has to become a whole new person, they might have things to work on, but not everything, and it doesn't always have to take that long.
Sorry for the slight hijack...