I am not for it.
From my experience, poly relationships have their issues. (To be fair, monogamous relationships have issues too, no one is claiming otherwise.)
From my experience, poly relationships have problems and temptations beyond 'normal'. Mind you, that observation doesn't have anything to do with the people involved, in an of themselves. However, the 'scene' usually associated with poly relationships has a lot of temptations which can cause problems which monogamous relationships have a greater resistance to.
To explain...the issue I've seen from poly relationships is that the assumed structure is much more open to abuse. Culturally and socially, in a monogamous relationship if one has relations with another, it's considered 'cheating' and (in theory) is Bad. However, in a poly relationship, it's much harder for others to know when someone is 'allowed' or not, which means it's far easier to conceal such activities.
In addition, in a poly relationship, boundaries can be (can be, not are- I have seen several very stable poly relationships) pushed too far without anyone realizing it. By the time those boundaries are broken, often it's impossible to go back.
In short, my main issue with poly relationships is that they have less resilience and lesser boundaries to keep members together, which can lead to more issues than in monogamous relationships and less stability.