Mephits have had significant changes through the editions. In first they were lower planes creatures made from local materials and used as messengers and weak creatures for magic-users to summon. In second they had two versions. In the Monstrous Manual they were still from the lower planes and had a few more types. All were evil and still used as messengers. 

Cursed children.

Mephits have had significant changes through the editions. In first they were lower planes creatures made from local materials and used as messengers and weak creatures for magic-users to summon. In second they had two versions. In the Monstrous Manual they were still from the lower planes and had a few more types. All were evil and still used as messengers. In the first Planescape MC there was a much more detailed version, though oddly they were listed as neutral. In PS mephits were elemental material drawn to the lower planes and shaped by a combination of Monster Summoning VII and Contact Other Plane, which makes sense as Monster Summoning didn't access other planes in that edition. They were disposable minions easy to make as they tended to die quickly, especially those made to insult other fiend lords. In third they became something very different- outsiders that lived on the elemental planes. They lost their cruelty and reason for existence. They became tricksters like elemental fey (and shrank for some reason).

Sean K. Reynolds released a pdf entitled File Off the Serial Numbers. It is a brilliant little work on using monster stats for NPC humanoids (representing classed people as well as cultists who gain power from supernatural sources). When I was looking through the first edition Fiend Folio, the monster that popped out at me was the mephits. And my first thought was, "They would make great cursed children." It would be child's play to modify the spell Create Darkenbeast (from Realms supplements in the various editions), one that is 6th level and affects only 0 level characters (that would be commoners in 3e). A wizard, elemental prince, demon lord or arch devil could have great use in turning lots of people into highly destructive minions. And that is what mephits are. Even without trying, fire mephits would burn down a town and the fields or forests surrounding it. Lava mephits work best in castles and other stone structures, burning through and destroying them in a matter of hours. Frost giant shamans might be able to turn slaves into frost mephits by infusing them with winter spirits, thus making scouts and shock troops when attacking human cities.

So what if the mephits are individually weak, especially in third edition. Their creation by cursing children means every slain monster is a dead kid. How many parties would be willing to do that once they know the consequences of their actions?