While that's an incorrect assumption (
http://us.battle.net/d3/en/profile/IlludiumQ36-1420/hero/1668575 ), I can see why you made it. The bolded statement is true nonetheless. Ball Lightning can be a curse when a reflect pack is just beyond a cluster of trash mobs. The ball(s) travels forward and continues to proc damage to all enemies within its range. Gloom doesn't proc on projectiles (or traps) fired before activating it - hence the fall over dead.
Your statement further illustrates the problem with reflect in the current state: Shadow Power is a must have skill, even at the
lowest MP level. Reflect is the limiting factor for DH build diversity, something Blizz has been promoting with the culmination of the last patch.
Reflect as a monster affix is completely unbalanced relative to everything else. It's as if it doesn't behave or scale in conjunction with incoming damage per each MP level. The amount of damaged returned at MP0 seems the same as any MP level.