this is, i think, a de facto model - people call themselves poly but either consciously or not end up in relationships where one person is in more relationships than another, sometimes for extended periods of time.
i'm not sure what the problem with this is, aside from some overly egalitarian theory that all people should have equal power at all times... which of course isn't how things work (and afaic, wouldn't be much fun if they did work that way).
to me happy relationships are ones where people a) know (consciously or not) what they want, b) have the freedom/ability to get it, even when it changes.
people will want different things from each other, which is a good thing.
that said, obviously there is an issue if one person doesn't actually want to be poly (or monogamous, for that matter) and agrees to it because they care more about trying to make it work with their partner than they do about holding on to their boundaries. sometimes that means that boundaries shift, and sometimes that means that one person is unhappy. but that happens regardless of the label(s) applied to the relationship.