That's a good question, perhaps I can start the discussion this way.
First let's note three things:
1) Divine Auras require a particular resource.
2) Divine Auras cannot be activated with the Quick Cast marker
3) Divine Auras have a limited radius of effectiveness.
Taking each individually:
1) The fact that they require Valor to activate means that you have to have generated Valor before activating them. This means that, even in an Ideal world, you cannot activate an aura before turn 4 and most likely it will be turn 7-10 before you can activate the least expensive aura.
2) The inability to use the Quckcast marker basically means it takes a Full Action to activate an aura. You cannot attack or cast a creature on the turn you wish to activate an aura which means that the benefit you gain from the aura would have to outweigh both of those options. Coupled with this, your paladin only gets 2 actions himself a turn, now those can be mitigated with spawnpoints and familiars, but it's important to note again that the action it takes to activate an aura must be worth more than any other action that turn.
3) Auras offer a nice little perk to the creatures in the Paladin's zone. The emphasis here is on "Little" and "In the Paladin's Zone". For the cheap auras the perk you get almost never outweighs the cost in action and valor. One of the main reasons for that is that only the Paladin's zone gets the benefit. The big aura takes a LOT of valor and still only hits the Paladin's zone (with a much more game changing buff).
There are a number of tweaks that could be made to the Aura system, but each one of them catapults Auras into the "You can't win when I get this activated" zone. Believe me, a lot of iterations of the Paladin in playtesting were massively overpowered.
Auras are in a niche spot at the moment due to their special resource cost, the action economy and the small scope of benefit. To put it succinctly: I always end up having something better to do on my turn than activating an aura.
