Tartarians in 3.17

[Endoperez]

Pillaging causes lots of unrest. You don't want keep the Tartarian-summoner in an uncastled province where he and his boosters would be vulnerable, which means that province is also a place for recruitment, and unrest over 100 will prevent that.
It isn't a huge deal, of course, unless you have dozens of Tartarians in your capital and you really want those capital-only Arch Theurgs or something.