I think the "can communicate with poor vision" is the most important as its dark in about 50% of the day when sign language doesn't really work.
Not to mention that you first need to call the other's attention to start communicating with signs, possibly by making some sort of sound so they look at you.
The second most important benefit is (IMHO) that you can speak while you are focusing on something, e.g. waiting for the prey to come closer while everyone holds their weapons.
