Very interesting topic - only thing I can think of off hand that you didn't include would be long term plans.
This kind of falls under what you said already. When I first started dating my boyfriend, when I met his wife she made some comments that made me think - when you date somebody short term, or casually, it can make a short term impact on your life, you deal with NRE, and an upheaval that can be a bitch. When that turns into a long term relationship (and you're already married) you're also potentially making a long term commitment to spend a certain amount of your life with somebody other than your spouse.
I became much more aware that since my boyfriend already had a long term relationship with another girlfriend who he sees once or twice a week, that becoming long term with me meant 1 additional day a week for up to forever, that he wasn't available for his wife. I've really thought about what that means for her, and what that'd mean for me if my husband got into one or more long term serious relationships and the impact on me if he was a hinge.
Another reason long term impact of a hinge came up for me is because there's a chance my husband Adam is getting a job in another state. If he gets it (and my boyfriend and I keep dating) I'll factor in the stress of me traveling back here to see Brian - not so much for my relationship with Adam, I think that's be fine, but it has to alter his schedule with his other two important people, and could cause stress on him as a hinge. If Brian visits me, as it stands Adam never sees Brian - if it becomes a LDR he becomes a guest in our home instead of a visitor if he travels to me, and I'd sleep with Brian instead of Adam during that time, and there'd be some group time - which would certainly alter the dynamic. Also if Brian visits me it alters his schedule with his other two important people, and again, could cause stress on him as a hinge. If I visit him it'd also involve a longer time commitment that could stress his other relationships. It wont be once a week, but I think a few day visit either way could have a stressful impact on his life, especially if it cuts into his vacation time that could be used with other partners.
I feel for you with what you're saying - guilt can be such a driving force, and I work really hard to keep it not being a factor. I'm sure if I communicated more with Brian's wife I might feel more guilt as the arm of that hinge, so maybe I am lucky that we don't talk so much. I find it pretty easy at the moment to balance all the things that need to be balanced, but I worry if I add any other relationships or changes in that it will become much more difficult to be a hinge.
Happiness will never come to those who fail to appreciate what they already have.