While keeping what during a long rest makes sense, especially if you are in a hostile environment, most of the time you don’t need to keep watch during a short rest. As long as at least one character is alert during that time, or alarms or other preventive measures have been set up, the party can focus on other things. Keep in mind that random encounters and ambushes can still happen during or immediately after a long rest, effectively negating the benefits whenever you enter combat. If you are in a hostile area, bandits can patrol around, monsters can catch your scent, and other dangers can investigate.