I'd agree that most just add Poly to their current doctrinal stance and don't move. Speaking for myself, that's hardly the case. My staunch Reformation/Presbyterian/Predestination flavor of "Poly" is in fact largely a result of accepting polygyny. It has to do with accepting the Sovereignty of God. For me at least, understanding that greater Sovereignty flowed directly from understanding roles in marriage and the fact that the man is in charge, modeling our LORD. I had wandered around mostly reformed venues of worship and generally preferred them, but I hardly knew why and I was a very weak "Calvinist."
Knowing that the organized churches would probably never accept polygyny, I had also wondered what would constitute the correct way go go about forming a new church. That became eventually, a "new denomination" but I still didn't have a handle on it.
As mentioned before, I am a "cessationist," which says I don't believe people heal one another (though it would have to be with God's power), speak in an "intelligible" tongue or write scripture. Similarly I don't think people hear God speak like he spoke to his prophets, or to Moses. No one hears God call them like Samuel did in this day and time.
Combining God's sovereignty with Romans 10 which states a preacher must preach for the Gospel to heard, and that preacher must be sent (it's interesting that Romans 9 is a nail down chapter for "Calvinists"), I began to see that there must be some endorsement from God, either second hand or first hand for there to be a sending and that sending wasn't going to be me waking up like Samuel, and answering God's voice.
Thus we need some sending to be a preacher, and we need a preacher to have a church.
In order, I became a believer in polygyny as a fully acceptable form of family relationship and marriage practice.
Then I became a cessationist. That's sorta unrelated to believing as I do on marriage, except for the fact that having become the way I am on marriage, I was reading the scriptures constantly to verify my belief and that's how I arrived at cessationism.
I encountered the "Mean and Harsh" Brandon Kraft over at "Theology Web" ("Dee Dee Warren's" site) in the course of my many frustrating and unprofitable debates with Dee Dee and her posse. Brandon sorta rubbed my nose in Isaiah 45 and I (being somewhat more disposed to accept authority structure) realized that God did author absolutely everything and I became a "Hyper Calvinist," which I find utterly acceptable as a polygynist. I am in fact very surprised that more polygynists don't see this, since they do accept (as male practitioners) their dominant and "unequal" role in the home. I guess they just want to stop there.
Roll it all together in a wad, and I take the position that Yes, we do need a New Denomination and/or church but we can't just strike out on our own any more than a woman ought to be without what some call a "covering."
I firmly believe that I have in fact been "sent" in much the same way that Luther was during the reformation, and more recently that Gresham Machen was sent to form the Orthodox Presbyterian Church. Being the "Jonah" that I am, I'd much rather hop a ship going in the opposite direction. You can turn my "sending" into "sitting" in the pew if you find for me someone else who has been sent and believes in Poly. They just have to be sent.
So far no one can explain to me without the extant gifts of the Holy Spirit being evidenced through believers (gifts like healing and prophecy), how someone is "sent." They just go silent and claim various reasons like "I don't want to argue." Here's the problematic text:
For whosoever shall call upon the name of the Lord shall be saved. How then shall they call on him in whom they have not believed? and how shall they believe in him of whom they have not heard? and how shall they hear without a preacher? And how shall they preach, except they be sent? as it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things. But they have not all obeyed the gospel. For Esaias saith, Lord, who hath believed our report? So then faith cometh by hearing, and hearing by the word of God." (Romans 10:13-17)
When someone can explain to me how it is that someone is sent, and show me someone who was sent, if I am convinced that their explanation of sending is correct, I will sit in at that man's feet or in his pews.
Polygyny has caused me to rethink what I have been told in every area of my faith. Some answers I have been given all along turn out to be good ones in my estimation, but I largely adopted them in the wake of accepting polygyny. I did not adopt polygyny in doctrine, and pin that tail on my other beliefs. It was in fact the other way around. As purposed by the LORD of course.
What I do see in a lot of poly people is precisely what you speak of, namely, they just add poly to their flavor and go on. I regret to say that most of what I see as a "flavor" is that they, the poly adopter, are on top of the pyramid in all things.