Yep I am with you. JSbin is great if you are code savvy/comfortable, otherwise it's hard when other users need to work off of a template.
Regarding Marketo's native email editor, I am told there is a pretty significant update coming to it in the near future, which has been a long time coming, in my opinion. Based on what I've heard, we should expect this sometime before the end of Q1, so hopefully that will make life easier soon!