There's a conversation thread that sparked my interest recently regarding the idea of "secondary" partners not being able to come into existing relationships as equals. This was seen as a negative, or something to be overcome by the metamour not directly dating the new person.
While not an adherent to hierarchical poly myself at this point, it can be useful to recognize that there _will_ be differences in relationships based on many factors, one of which can be longevity and shared experiences. Someone that plugs into an existing relationship of any length, and expects to be on equal footing immediately, is likely to be disappointed. This certainly doesn't preclude the possibility of growing into that space, but some patience in growing those relationship bonds is likely to be hugely appreciated by preexisting partners.
Each relationship requires some time to find its own footing and depth, which is often confused with the intensity that marks NRE. Take the time to give all concerned an opportunity to form a firm foundation of mutual appreciation and shared experience, and the sense of equity can naturally grow within a structure that has that as a goal.
If you, a person looking to gain a new partner, are entering a situation where the pre-exisiting couple isn't interested in getting to a place of equity, let's hope that it isn't a goal for you either! If it is, bring it up early and often with all concerned. If that doesn't appear mutual across the board, the time to get out is now. Banging your head up against a metamour that is zealously guarding "their" partner is a great way to learn to hate poly!
There are many who enjoy less intensive relationships involving fewer overt responsibilities. I'm involved in several of those, where the relationship goals don't currently include things like cohabitation, raising kids together, or being day in-day out partners. There's nothing that says that may not come up in the future, but for now, this is what we are all interested in and agreed upon. This doesn't make them less than, but they are different from the relationships that I share with my live-in partners. That's my reality as it currently stands.