@ DarkKatGirl: I scratch behind the ears, you purr?
Act 1 mercs are rather powerful, especially when you note that any merc can use any weapon that is not class-specific (and act-5 mercs can use barb-specific hats and belts). This means that the rogue can wield a Doom rune-word Thunder Maul (after being made ethereal and made easier to use with Oil of Skill) so she can have a Holy Freeze aura. Also she can use a Faith rune-word bow, so she has a Fanaticism aura. Etc. Act 1 mercs use a skill depending on the difficulty they're in. In Normal/PoC they always use fire/cold arrow. In Nightmare/LR, they always use frozen/exploding arrow. In Hell/CGS they always use freezing or immolation arrow.
By contrast, the act 2 merc uses the aura determined by the difficulty where you hire them. Thus an offensive merc from hell always uses fanaticism.
Act 3 mercs have a set of spells that they use determined by their level (you're 24 and he's 23 then he won't use Blizzard yet.)
Act 5 mercs have a conviction aura, but it's fairly weak.