mangos/mangos — comment on commit 2068b50e0912ec89b49b78be2b348cfc9535b629
File:  line 0
By Schmoozerd:

actually - not really..

The only reason that there is this script-effect spell can be to cast the spell multiple times.
Depending on the way the implement script effects (likely some lua based scripts) this can be much easier than to create a system to apply auras with a stack-amount (or to add an exception to such an existing system)

We don't know their tools, but we can be very sure that for them adding a bunch of spells to work with each other is very easy;
example a player controls a npc and casts a spell to a trigger-npc which should give some quest-credit.
Then blizz might find it most natural to cast a spell from trigger-npc to controlled npc, which casts a quest-credit spell onto owner
(because the trigger "knows" directly only the caster(the controlled npc) - and the controlled npc knows the player)

As a matter of fact, this very spell here could have been implemented as 11 entries into spell_scripts by database table, no need to actually do anything with c++ here.
(I didn't realize this on first look, because I assumed the spell would have been cast appropriate times, and not a fixed number of times - which would not have been doable in our db-script system)