Yes I made a list, 11 positives for a mono in a poly relationship, it's under favourites on my blog. I even subsequently added a couple. It is my list though and some of the points are specific just to me.
But you aren't mono you're poly. In reply to another of your threads there is a site on yahoo groups, called livingpolymono that could be good for you. It's for polys in relationship with monos, which actually might be more your thing. One you have a relationship with a mono metamour to sort out and two I am beginning to wonder if your husband is actually poly. There is also a polymono site for monos in relationship with polys, so you could take your pick. Many of us belong to both.
I know this is sort of the wrong thread to be saying this but I'm multi-tasking here. One big positive for me in polyamory is that it seems to bring problems in relationships to the surface, and that's definitely what it's done for you. A whole hornets nest in your case.
The first and most basic thing that strikes me with you is that you seem to be putting your husband's and his girlfriends needs way above your own. You keep wanting to back away from communicating here because it upsets them. Sometimes things need to get worse before they can get better. Sometime you need to upset other people in order to be true to yourself. A lot of resentment has built up over the years and dealing with it will be messy but you have to deal with it before you can be free of it.