Actually, you can list several different names in the DirectoryIndex directive.
If you list
DirectoryIndex index.html index.htm /noindex.html
then the server will display the first file it comes across in the list. If you have software that names your files with .htm instead of .html, this would make the server display index.html if there was one, and if not, look for an index.htm, and then look for a noindex.html file in your root directory, before giving up and listing all the files in the directory.
The advantage of having a /noindex.html file is that if someone snoops in a directory where you forgot to put an index.html file, you can display a page that says “Private. No girlz aloud” like the little boys’ treehouse.
Not letting people know any more than necessary about your file structure is called “security by obscurity” and it’s not the most effective security tool, but it can help a little, and at the cost of virtually no effort.