To be fair it all started with "controlled Kerry" because the dev tea didn't really have many art sprites to work with and with new events it was easier to toss a controlled Kerry than to design a new boss; and the idea kinda got reused not for the lack of sprites, but rather to change the game a bit imo. Think about it like this, you always fight monsters and some even get to be boring because you see them all the time so fighting a main unit is quite refreshing e.g. the AW orbs dailies and the new AW fairies map on weekends