why does bones theme call the_excerpt function with parameters?

This is a bug in the theme. The real function does indeed not use the parameter:

/**
 * Display the post excerpt.
 *
 * @since 0.71
 * @uses apply_filters() Calls 'the_excerpt' hook on post excerpt.
 */
function the_excerpt() {
    echo apply_filters('the_excerpt', get_the_excerpt());
}

PHP will just ignore it, but the unnecessary gettext call should be avoided. Write a bug report.