A few usability question about git diff --cached

Hi all,
I'm having and interesting discussion with an happy mercurial users
which started reading the git documentation. He raised aquestion that
I'm not able to answer:

$ git diff --cached

(Without —cached, git-diff(1) will show you any changes that you've
made but not yet added to the index.) You can also get a brief summary
of the situation with git-status(1):

Why do we have the option "--cached" and not "--index"?

I believe that, just reading the documentation, is not very easy to
fully understand the role of the index and then we have an option
"--cached" that refers to the index.
Is it a good idea to add and document an option "--index" and remove
from the doc the option "--cached"?

Thanks.

Regards,
-- 
Paolo
http://paolo.ciarrocchi.googlepages.com/