When you think about it logically the Tau trying to find a way to interbreed with humans is extremely logical. They lack psykers and know that they can't really fight chaos, the ethereals just suppress the information (and no the nicassar can't help the tau with this, that was always fanon, their psychic abilities are very limited in scope.)

But breeding a new caste of Tau/human hybrids with brighter souls that are still vulnerable to ethereal mind control....well....isn't that an idea