Creating a "new" MXD is the same as just closing a currently open version. There really is no reason to have ArcMap open if something isn't open. Unless you save it to disk (as "untitled" or as a new name) it is just a shell waiting for you to do something. The rest is just semantics.
Just out of curiosity, why would you want to have to use File-Close....then File-New or File-Open (more clicks), when the later two does the File-Close anyway. Is there a reason you would want ArcMap open......without having the option to start adding data?
So, technically from a user standpoint, the way it is setup is more efficient, and probably was due to users requests from many versions ago to eliminate the extra clicks.
As you mentioned before, you can have several sessions open (and is an easy way to copy items between MXDs). If you were wanting multiple layouts in one project/document (which used to be available in ArcVIew 3.x and prior), look at the new ArcGIS Pro | ArcGIS for Desktop which was written from the ground up and is 64-bit (versus ArcGIS Desktop which has been around for a while and 32-bit). That may have more of the features and GUI that you are wanting.
EDIT: BTW, if you are having issues with an MXD freezing....try the Save As option....might be a "glitch" in your MXD.