I think there are two problems with the Infernal world spell:
1) It's pretty useless for the Infernals. What I've seen happen in my games is that with their homeland on a far-off continent, they use the spell to take over one of my cities. I demand that Hyborem liberate the city, he refuses, I declare war and march in to retake it. Spell wasted.
2) Unlike other world spells (e.g., Stasis, Arcane Lacuna, Raging Seas, Worldbreak, River of Blood),
Hyborem's Whisper adversely affects only a single opposing player. Yes, you can usually quickly retake the city, but if it was a city with one or more National Wonders, it can be really difficult to recover from. If it's a city in which you had several levels of the Altar of the Luonnotar, you might never recover, since you might not generate enough Great Prophets at that point to rebuild it.
Again, although some of those other world spells can be unpleasant, they at least adversely affect everyone, unlike the Infernals' world spell.
Gekko's right that in MP people can just agree not to use it, but that wouldn't apply to the AI in SP.
We had some discussion on this over in the RifE forums, and if development on that mod had continued there might have been a new world spell for the Infernals. Krieger66 came up with an idea
here, which Valkrionn suggested modifying
here, and Krieger66 suggested a final modification
here. Just putting it out there for your consideration, but it might be more than you want to change.