So which is better -- partnership or companionship? I polled a couple friends of mine (all poly) and they seemed more inclined towards companionship. Is this a general poly attitude? It seems reasonable. The "partnership" position, as propounded in books like The Heart of Marriage, seems to correlate with a desire to have your entire life enmeshed in a relationship with another person. The danger is that you won't have any life of your own left, because you've tried to meld it all into your partner's (or "the relationship's").
However, this danger is not exclusively a danger of monogamy. If you have two partners, it is just as possible to try to bury your own personality, your own wants, in the desires of your (several) partners).
Conversely, monogamous marriages don't necessarily have to follow the "partnership" model. This was part of the theme of Open Marriage by the O'Neils. It is possible for two people to be married to each other in a loving, enriching relationship - at still have lives of their own individually.
But - for some reason, this seems quite unappealing, even threatening, to some people. I've heard companionship called "unsexy". Is "mere" companionship not exciting enough? Do they fear their beloved will leave them?
My friends and I discussed this. I told them my ideal has often been to have a group of companions I care about and with whom I have adventures -- but who allow me to also have adventures on my own; and who will listen when I relate these "other" adventures to them. I was surprised to discover that this is also the ideal of some of my friends. And yes, sometimes when people have adventures "elsewhere", they don't come back.
But if you choose the right companions, I believe they will. That is my faith, my trust in the people I love. And even if they don't come back physically, I will always feel connected to them spiritually.
I would probably not sacrifice my happiness and the happiness of my partner(s) for the sake of an abstract entity ("the relationship"). But there is such a thing as "the relationship" as an entity in itself. So what I actually believe in, I guess, is marriage as a partnership which is not all consuming, a good thing but not the only thing I live for. The ideal for me is neither "just" companionship nor partnership alone, but some blending of the two.