And as for the original post, I don't have the wherewithal to talk to all the points (great post, though!), but I know that in my partner's case, he "goes deep" with both me and his OSO. He gets close to friends and family as well, and if he had the time and energy to "commit" to everyone he loves (friends, family, etc.), he probably would. He's had a hard time in the past when he can't be there for one or the other of us as much as he'd like.
The boy's got a big heart, which I love, but which pretty much got me into a relationship structure that I never expected. Funny that.