I'm so relieved by everyone's posts! It's great to hear all these different takes on how to understand and get along with metamours, or how to just let them be who they are regardless of if that's friendship material or not.
In my own experience, my metamour and I are friendly, but nothing really beyond that, and it works for us. He's a nice guy, I see the things my partner could love in him, and I see things that I personally don't value, but I can appreciate him for being kind and loving to our partner. When I first entered our V, I figured we should play it like one big family: I invited my metamour out a few times just me and him to get to know each other. What it really came down to is that he's really reserved and I have a hard time connecting with men. We have similar interests, just nothing really to get us to a deeper level of connection, and there are parts of him that I don't like, but I'm not the one in a relationship with him, so it's really none of my business. I think the trick to being in a poly relationship with somebody is to accept their metamour for what they are, and really expect nothing else. Our partner is our most basic common ground and even if we don't like each other, or really really enjoy each other's company, our needs and desires bottle neck at our partner and we should respect each other to keep the love flowing.
So, I guess the core of my point is that whether you love or hate your metamour, as a poly person it's your responsibility to make room for and respect their relationship, because an unhappy hinge makes and unhappy V... or N or Q or whatever it is you're into.