The Offensive Auras Skill Tree is primarily aimed at improving existing combat skills, but also provides some unique attributes that wreak havoc with the enemies. For example, Holy Fire will periodically hit any nearby enemies with a blast of fire, while Thorns returns damage to the monster inflicting it on you. Where Offensive Auras really shine, however, is in multiplayer action. A multiplayer team without a Paladin versed in these skills will be at a considerable disadvantage.
No Offensive Auras use mana, so it is fine to always having an Aura active, as any Aura is better than no Aura.
A Paladin cannot use more than one Aura at the same time. Different Auras can be stacked by multiple Paladins.
The Act II Desert Mercenary also uses Auras which may stack with a Paladin.