If I would date someone again would really depend on why the ex and I broke up. If it was a situation that can change then revisiting a relationship would be great. If it was something that's not going to change, then there's no point in revisiting because you already know that there are some irreconcilable differences.
Granted, polyamory makes some of those differences that don't change moot as well. Two people who don't do well in a live-in, monogamous, completely closed relationship may not work but when you add in openness they can make it work.
So, it really depends. I wouldn't see a big issue with it unless it was a truly terrible thing that caused them to split the first time.