Yeah. The yeast will die or become dormant at the very low temperatures and thus affect your beer. The jacket can only do so much. That is why the immersion is there, to reach the desired temp. The jacket then maintains the temps for a little while until the immersion unit kicks in. The jacket reduces the need for the immersion unit to run non stop. Further, the immersion unit is not a heater, so it is useless in very cold environment.