Information about unit status is not the same thing as information exchange. An overlord senses all of his units, but can't talk to them directly without Thinkamancy (see an early page in book 1, when Stanley senses a loss of spidews unit). The same goes for casters, who create units I guess. Why would anyone use hats for sending reports, if an overlord could transmit all information and orders between his units?
Well, it would still be useful for the main purpose for which we've seen it employed, transmitting information and items between different sides. (I agree with your point about Wanda and her knowledge about Decrypted units, though.)