I have no idea how prevalent that might be--I've never looked to get involved with somebody who's poly and married (or has a long-term primary). It could be the most common sort of dynamic currently manifesting, though I think that would be a sad state of affairs.
I find it sad because I think it speaks to a fundamental problem in the primary relationship--and I have to wonder why'd they'd even consider adding other people to their lives when their primary relationship isn't very strong. Look, folks, if your existing relationships aren't good--and that means functional and strong on every level--then adding more relationships is not a wise thing to do!
I don't think I have to set any boundaries with regard to that, simply because I'm never looking to replace my wife. I married her for a reason and that doesn't change simply because I may be involved with somebody else. Should the laws ever change and we could add people to our marriage, that'd be cool--I just have no interest in trying to replace my wife.
As long as my wife and I are taking care of "Us," adding more "Us" combinations isn't going to be a problem. Setting boundaries on those other "Us" combinations can then be seen to be rather silly, I think.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.