Sauromantian Soothsayers: are they there exclusively to feed my evil blood rituals?

[K]

Soothsayers make fine researchers since they prevent bad events, are very cheap, and they can be recruited at any castle without needing a lab or temple.

As for magic, they synergise well as Body Ethereal and Luck casters for hydras assuming you put a Poison Ring or something on them.

But yes, they do seem designed to be Communion slaves. Witch Kings get W/D/B/N, so you can Communion up for the big magics in those magicks, but a few boosters/boost spells and a Communion can get you the interesting Astral magics like Will of the Fates.