Save as Draft functionality
What would the main use of this? If a content is kept as unpublished it is similar to saving the draft. It will not be visible to anyone else except the author and admin and any one else you have given permission to view the same. So this in itself is a good option. The save as draft module does the same thing it creates a button to uncheck or check the publish unpublish checkbox. 

If you are looking for a workflow kind of feature then workflow module would be a better option. where you can define different workflow status and give different users permission to access the node in each workflow state.