Wow, I really like "monomour"! That is awesome!
I have dated several couples and here are my feelings on how their relationship affects me.
If the primary couple's relationship isn't going well, it always affects me negatively. I derive absolutely no benefit from difficulties they may have in their relationship. I have no desire to be come a "confidante" of my partner at the expense of their primary -- to do this would feel unethical and extremely uncomfortable to me. When my partners are having problems I try to do all I can to tell them I support and love them both, but I don't try to interfere. It's a very tough position to be in.
I DO feel some responsibility for their relationship in the areas where my presence may affect it. For example, if they need extra time together and I can give them the breathing space to have that, then I will generally step back and give them the opportunity.
I also feel it's not my place to act as a communication buffer between the two of them, and while it's ok for them to talk to me about the other person if it's something we all know about, I don't ever want to be privy to a "secret" about the other partner. This is different, by the way, from things which are private between me and my partner (or partners, since I'm dating each person in the couple in one case, one person in the couple in another case).
I am happiest when my partners have a great relationship, but I know that I can't facilitate this for them. However, if I am ethical about what I say and do with them both, I can at least feel good about my role in their relationship.