I am personally FAR more comfortable with a "closed" polyfi relationship. The difference happens here for me:
When you have a primary couple, where both either see the same third/fourth/fifth person, or where both members of the couple have flings/FWB/whatever on the side, there is no reason for it to be closed off. You have FWB. They don't require a whole lot more maintenance than a normal "friend", and yay! more people to fuck. When sexuality is as important to you as it is to me - that's a big deal! lol The main point here is you can have a TON of friends, not even necessarily with benefits, and maintain your relationship with all of them.
When you have a primary couple, and you begin dating another seriously - time is NOT an infinite resource. Take my relationship. HMA and I were in an open relationship. He got booty-called a couple times, I took a few girls home, it was all good. It didn't infringe on our "couples" time at all. Then, we added Anne. She is dating both HMA and I, and is considered my equal in the relationship. The amazing amount of strain on all three of us to maintain equality, honesty, communication, happiness, ensure everyone's needs are met, blah blah blah - is HUGE. It's mind boggling. I can't imagine adding another "equal" partner. NO WAY. Three is hard enough!
As far as my triad goes - when you're completely equal, and have 2 other partners who complete you, are your best friend, and give you everything you need - why in FUCK do you need more?! There comes a point where I feel it just gets greedy, lol. I'm not bashing anyone in a quad or anything, it's just how I feel. You don't need more, more more more MORE. Polyamory, to me, isn't about collecting lovers. It's about building a strong relationship with more than one partner. And there come a point, time-wise, where that just isn't possible with a large group of people. It's just not. And it comes across as selfish.
For me, I am either in an "open relationship" where there is a clearly defined Primary Couple, or in a "polyfi" triad/MAYBE quad where everyone is devoted to one another monogamy-style. Make sense?