rev-parse, unknown arguments and extended sha1's

Is git-rev-parse supposed to echo arguments it doesn't understand ? It currently does, but git-checkout-script seems to think it doesn't:

                rev=$(git-rev-parse "$arg")   
                if [ -z "$rev" ]; then
                        echo "unknown flag $arg"
                        exit 1
                fi

Also, is there any reason why the extended sha1 notation is restricted to the scripts and not used in the actual plumbing ?

skimo