The main reason for cutting the branch is the thorns, they are seriously vicious and it is impossible to get at the berries without being stabbed. I have seen one method of picking where you put on heavy rubber gloves, and run your hand down the branch squishing all the berries and pushing the juice into
bucket you're holding under the bush.
As to deer, they do not bother them here the thorns are to much of a deterrent, they even eat the rosa rugosa first. Not much eats the berries either they are so sour even the birds avoid them until there is nothing else to eat, they can cling onto the bushes until past Christmas.