Military units are broken into discrete classes to define their role and function. With religious units, apostles do all they heavy lifting, with role and function are defined by promotion and (perhaps) an enhancer belief. What you do with a Debater, Proselytizer, Chaplin, or Martyr for is all more radically varied and nuanced than what you get from most military promotions. Imagine if a cavalry, melee, and anti-cav unit used the same iconography.Military units are certainly not distinguishable based on their promotions, like a horseman with Coursers and a horseman with Caparison, although units can be renamed, maybe Apostles can too.
However, if you combine units into a corps or army, that is made distinct so that players know when they're about to collide with a bunch of units with +10 combat strength. A Debater gets +20.