Should you write with a cause?
I think addressing social issues directly is more a part of literary fiction than genre fiction. I think genre fiction has a history of playing with ideas, but I don't think it has a place in main plot because I don't believe that authors should be preachy. That said, I think we all have our own sense of how things work, and I think that there is no way that an author can keep his/her perspective out of their stories and characters.
I don't think we should purge political discussions from books just because it isn't appropriate, or it upsets some. It's easier to take some of these concepts through fiction, where feeling a connection to the characters drives the readers ability to consider some of the issues of the world. Readers though, I believe, do distinguish between the issues in a fantasy world and the real world. I knew a girl once who's favorite character in a novel was gay, but in real life was a staunch opponent to gay marriage, which led me to think that people maintain this separation.
But that doesn't mean that causes should be anywhere close to the driving aspects of plot or character.
Characters should drive the stories, and any issues they faces should be same as we face in our world: directly relevant and impacting to relationships and decision making. But the character is what it's all about.