The human race would be most useful to the Umiak as they apparently are resistant to Loroi telepathy, not to mention that two of our worlds are of the type preferred by them. As far as we can tell the Loroi treat their allies and subjects much better than the Umiak do but are not afraid to commit xenocide. The emperor might decide to destroy humanity before the Umiak can enslave them. Then the Barsam and some other Loroi allies will come to humanity's aid, the newly formed alliance will also liberate some of the races subjugated by the Umiak.