I'll give this a shot, since I've been in both a "merged" and "unmerged" poly situation. Currently I think I'm sort of straddling the two worlds, as I live with and am committed to my children but do not live with any of my partners.
There are different stages, as I see if, of our relationship lives -- having children and their developmental stages is an obvious part of this, and we see this in monogamous relationship developments over time as well as poly ones. I suspect that when families are young and children are small, there is more of a need for merged, even "closed" poly relationships because taking care of our young is a primary concern for humans.
As our families grow and develop, we do the same, and sometimes there is a need for "un-merging". Sometimes this has to do with the age of the people involved, but sometimes that's not a factor -- life experience happens at different times for different people.
If you have certain needs from your relationships then it's important to be able to identify and express them to potential partners. It's also important, I've found, to keep in mind that change is going to occur on many levels.. our families will mature, our partners' needs will change, and our own as well.
I know that's not very concrete advice, but it's really the best I can offer from my experience. If I'm not always checking in with what I need and feel I tend to feel bewildered by the changes around me or get lost in them and not tend to my own needs.