I think "for the horde" is a nice diplomacy spell

, especially when playing on map scripts like continents, where players are separated by ocean.
By enabling the early contact, the spell slightly reduces cost for technologies and provides nice clues for a next target.
Also, although I have never tried these strategies, it sounds nice to cast the spell after involved in war against barbarians with a large map, where plenty of them and thus barbarian villages should remain.
By doing so, it seems you could pop up decent number of villages, and could take control of many goblin forts as well.
And if Illians existed in the game, it seems to be really nice to cast the spell + hire barb scorpion units from goblin forts during the Statis by Illians, allowing devastating attack on non-Illian civs.
However, I also believe that it might be nice to if the units acquired through the spell were free in maintenance.
It is quite sad to cast the spell, then start deleting the units from economical issue...
I mean, those barbarians had stood up "for the horde", seeking to help the player in some way,
but then, I should go and say "okay, I don't need you because you aren't worth the maintenance cost." to many of them...
And at last, sorry for being off topic!
EDIT: after thinking of it for a while, maybe that is why both leaders of CoE has either Organized and Expansive as their traits?
Organized would help reducing the city maintenance from mid-game, and also military state would be much more useful with reduced maintenance, while Expansive allows more cities and population from early in the game, thus more free slots of units.