Meh. I doubt there's much effect, personally. As with any part of the human experience, there's going to be a range of behavior exhibited. I suspect the number of folks who would do that sort of comparison shopping are in a minority.
To continue with your example, I've not ever thought "This one is much better at fucking than that one" and then spend less time naked with the latter. Srsly, if I'm pleased and happy with the relationship, then I engage in it fully and won't cut any part of it down because I could conceivably get better with a different partner.
And that's real world experience talking.
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.