Not necessary, no. But are they useful? Hell yes.
There are plenty of bodybuilders I know who have big backs without deadlifting. It depends on your goals, really. It's my favorite exercise though.
Some weeks I'll go for rack pulls instead of deadlifts, but I only do that if my legs are sore from leg day...in which case I'll do rack pulls to give them more recovery. But I saw the best gains in my back when I started to do deadlifts.