I've played through a big chunk of the game using Aeriel and Big Bootay as my hirelings. It seemed like having an extra sorcerer and an extra cleric would be most useful.
Just now, I've discovered another cleric and sorcerer - Friar Fly and Dark Mage. However, aside from being at a lower level, their specs are worse than Aeriel and Big Bootay, who have been given various stat buffs.
Is there any reason to switch hirelings? I could subject them to the same buffs, but then it seems like they'd be just as good as the current hirelings, not better, which isn't much of an incentive to go to the trouble of buffing them.

Many players pick up hirelings only for hard battles and dismiss them afterwards so they don't have to share XP and pay money. This way hirelings don't improve much and it makes sense switching to higher level ones.
For you there's no reason to switch hirelings now. If you get a quest where only robbers and wizards are allowed to go, you might want to add several hireling wizards, but for normal exploration you don't need new NPCs.