No index on the url_rewrite table

Question

Is there any reason why there is no index on the entity_id column on the url_rewrite table? Or on entity_id and entity_type?
The only index I see is on entity_id and store_id.
I understand that that usually a select is done on entity_id AND store_id but still...
Side question... are there any drawbacks if I add this index myself?

For the record, this is what show create table url_rewrite shows:

CREATE TABLE `url_rewrite` (
  `url_rewrite_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Rewrite Id',
  `entity_type` varchar(32) NOT NULL COMMENT 'Entity type code',
  `entity_id` int(10) unsigned NOT NULL COMMENT 'Entity ID',
  `request_path` varchar(255) DEFAULT NULL COMMENT 'Request Path',
  `target_path` varchar(255) DEFAULT NULL COMMENT 'Target Path',
  `redirect_type` smallint(5) unsigned NOT NULL DEFAULT '0' COMMENT 'Redirect Type',
  `store_id` smallint(5) unsigned NOT NULL COMMENT 'Store Id',
  `description` varchar(255) DEFAULT NULL COMMENT 'Description',
  `is_autogenerated` smallint(5) unsigned NOT NULL DEFAULT '0' COMMENT 'Is rewrite generated automatically flag',
  `metadata` varchar(255) DEFAULT NULL COMMENT 'Meta data for url rewrite',
  PRIMARY KEY (`url_rewrite_id`),
  UNIQUE KEY `URL_REWRITE_REQUEST_PATH_STORE_ID` (`request_path`,`store_id`),
  KEY `URL_REWRITE_TARGET_PATH` (`target_path`),
  KEY `URL_REWRITE_STORE_ID_ENTITY_ID` (`store_id`,`entity_id`)
) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=utf8 COMMENT='Url Rewrites'