The "address" field contents depend on a number of changeable factors, and may not always contain the fields you're looking for. You're right, though - it is the only field that can contain street information.
If you manually make sure you always have a street address when you're adding the location, you might be able to get away with making your own shortcode in your theme's functions.php or equivalent:
function ad_address( $unused ) {
$gm_location = GeoMashup::current_location();
if ( !$gm_location )
return '';
$address_parts = explode( ', ', $gm_location->address );
if ( count( $address_parts ) < 2 ) )
return '';
array_pop( $address_parts );
return implode( ', ', $address_parts );
}
add_shortcode( 'ad_address', 'ad_address' );
Written off the cuff, but if that's right then [ad_address] would give you the address without the last item.