Its the dirty cheaty back door tactic some dark elf players use to auto win every game by having the enemy army pinned the whole time and shooting them to death or manipulating assaults
Horrorfex spam is a crutch for dark eldar players that cant win other ways
Its RAW legal and valid, but frowned upon.
Also its fairly easily countered by mech armies, guard orders, monstrous creatues, and expert players
Edit: With only a 12" range, you're asking to get kicked in the vagina if it fails