what is minibuffer-completion-confirm's purpose?

[Chris Green]

In XEmacs, I can set minibuffer-confirm-incomplete and that will
prevent me from accepting a minibuffer completion that doesn't already
exist without confirmation.

For example, C-x b *Unknown* will prompt saying [no completions,
confirm] if the buffer *Unknown* doesn't already exist or C-x f
.bashtc will also say [no completions, confirm].'

In Emacs 21.3.1 and current CVS,

minibuffer-completion-confirm seems to provide that same kind of
functionality by it's docstring.  However, setting it to t doesn't
give me the desired behavior combined with emacs -q.  Is there another
minibuffer customization I can perform to get the promping on unknown
behavior?

Thanks,
-- 
Chris Green <cmg@dok.org>
 "Not everyone holds these truths to be self-evident, so we've worked
                  up a proof of them as Appendix A." --  Paul Prescod