I consider the preview of the markdown to be the essential part for new users opening a markdown file, and not the markdown editor. The current default is to open the markdown editor though.
I’m thinking that if one knows that one wants to edit, one has probably committed a bit to the action and wants a preview open anyhow along side working with it. Also, if you are working with the file that you know you want to edit, you are probably having more experience and will be able to find it, perhaps you even realized that should be edited due to viewing the preview first. Also, if you are a new users, you will be afraid of the file if opened in editor view but not afraid of it if opened in the preview view.
So, what do you think? Does it make sense to change the default to be in preview mode?