I did not it yet either, though judging from the performance of Soulgate i'd say - it might, possibly, be somewhat useful in an EA game if you go straight for it and have a lot of provinces with strong dominion. In MA you'll have better things to do with the gems and in LA, well, the autospawns aren't viscous enough to even slow the other nationals down.
Autospawn dominions were quite powerful in Dom 2, you could use your free spawns for some hyperexpansion early on and switch to summoned troops when they become obsolete.
Enchantment 8 and 90 gems don't agree with early expansion though.