User blog comment:Mathmagician/Why Dart Traps?/@comment-80.13.73.109-20120109172840

Because dart traps will hit all ennemies you can face. Lava cannot hit them all (the flying specter is just immune, as are burrowing monsters). Also, most flying monsters will not run into lava unless your character is in a specific location. Which makes the lava inferior VS burrowing monsters, flying monsters, and stronger against land monsters.

Note : this does not prevent you from using both lava and darts for house defence.