Aren't most jobs rare? At the top of my head there are only a handful of dragoons, there aren't that many red mages, paladins are more or less the elite guards for the sultana, dark knights are rare, machinists are just starting up in Ishgard, I don't remember bard being too popular in Gridania bc they don't believe in the power of song. I don't think its just a healer diversity problem but rather its supposed to be rare and above the class roles.