The problem with the current implementation is that IF the server configuration has changed it to something else, then this action breaks the server.

For example, assume that the save_handler is set to memcache:
Then session.save_path  would be: sess1:11211

If you change session.save_handler to files then sessions break because the file path sess1:11211 does not exist.

If you set the save_handler to files, then you must ALSO set the save_path to something reasonable.  To do one and not the other is broken.  

I choose to go the route of not doing either since there is not a good way to determine a reasonable file path.