I think the job customization comes from choosing your job. If you want to play a DD, there are half a dozen options for you, why choose one of the tanking options if what you really want is to deal damage? The DPS abilities that tanks and healers have, like your example of WHM, are likely so that those roles can still solo effectively.

That said, in the future you'll likely be given an option a lot like arcanists have, where the class branches into different jobs. So perhaps GLD and CNJ will eventually have job options to become something other than a tank or healer. However that will just be more customization from choosing a job.