I think all that matters in this discussion is that if it works for you, then do it.
This is a very interesting topic, and obviously one that a lot of people have strong feelings about. Certainly those who are presently in poly relationships. I hope everyone can rise above making generalized, negative statements. I am quite sure that wasn't Q's intention when he started this.
Personally, I don't think I could handle such an arrangement. It quite possibly has to do with my own insecurity. If I had a Dom that had another sub, especially around these parts where I know and care about many people, I would feel that He was just wearing a sign that says "lily just doesn't live up".
Again, that is just my thought. And I have never actually been in a poly relationship, so maybe it would be different if I was. But at the end of the day...I just don't share well with others. I don't expect that to change anytime soon.
I hope that more Doms/subs will share their experiences. Not out of defensiveness in regards to some comments above, but in order for those of us who are legitimately open and curious to understand. What makes this community great is everyone's openness, allowing us all to learn from one another.