I think the "friendship first" comment for many is simply a way of being comfortable with the other people. They get to know them well enough to know how their relationship operates and if they might be harmful to their relationship. This is probably mostly a newbie situation since it seems that as experience takes over this does not seem as important.
Another aspect of it is that some of these couples may be more poly then they think at first. We've developed many friends in the poly community in our area because even though we are technically swingers, we are very poly-friendly and have found occasionally that our relationships with others has been more then just a screw on Saturday night. Especially Mrs. WS. She gets more attached to those we swing with then I do. Interestingly enough, we've met several of Mrs. WS's high school friends in the poly community. Who'd of thunk it?
The trick is figuring out which is which.
Mr. WS