It frightens not only the enemy you crit against but all nearby enemies. It is already quite strong because every hit against a sleeping target from the close up is an auto crit, same works for knocked out npcs. It is already quite good depending on the setup. Also warlocks can shoot 2 projectiles/action with EB starting from level 5 and it raises the proc chance.
Scorching ray can also be a really good spell because it's a triple hit. Combined with the extra d6 from hex that spell can be murder.
