I'm sure it surprises no one that I don't subscribe to this notion. But that's not really what my post is about. What I began thinking about was this:
If one needs to advertise attractive traits to attract partners, and maintain them so as not to lose partners, then it would stand to reason that polyamorous people, many of whom are at least theoretically open to finding new partners, would be especially scrupulous about advertising and maintaining traits they think increase their attractiveness.
I'm not part of every community of polyamorous people, but the folks I am aware of don't seem to think very often in terms of "I have to maintain X, Y, and Z traits to keep my partners / attract new partners." I'm aware of people thinking in terms of how their traits interact with the traits of their partners, so that when there are problems, those specific problems get discussed; I'm not aware that "you've let your appearance go and I feel it's unfair" is a common problem. I'm aware of people thinking that they're responsible for continuing to pursue their interests and activities, so they don't become dependent on one person for all their social and intellectual needs. Maybe I'm wrong and poly folks (especially women?) are thinking a lot about maintaining their appearance in order to please their partners and attract additional partners, but if so I don't see it. I do see a lot of people concerned about their weight, but usually other reasons are given.
I'm sure that a lot of monogamous people also approach "coupled life" in one or both of the above ways, too.