I would agree that people coming in with their existing OCs have different agendas. For me it's about expanding and getting to know my characters better, rather than any real exploration or development of pre-existing canon characters. It's also absolutely all about the writing and characterisation, but I know that could be said of many players, regardless of the sort of character they play.

Playing both halves of a couple is an interesting situation. It's harder than I thought it would be, but I like it a lot. It makes it easier to portray how well they understand each other and their shared history. I think it works well for them because they have such a long history together, but wouldn't necessarily if they didn't. :-)