Initially I thought it was dumb to take the communication watch during the rat infiltration because all the rats were clearly able to talk with one another, but in retrospect it made perfect sense.
Kyle, Margaret, and Yuna were fully possessing their rats, and thus were capable of speech. Touka and Anri were using remote necromancy, and as such they couldn't talk as rats. Touka and Anri could talk to each other back in their original bodies, but they needed the watch to communicate with the other three.