No, they are not absolutely necessary, and can actually cause more damage. It's definately not uncommon for people to have more confidence that skill because of a flight simulator and how well they do in it. Then they go out and fly the real thing and crash and burn.

It's a good thing for learning the basics and orientation and ingraining muscle memory as well as trying out new maneuvers, but nobody should be under the false impression that if they can fly in a sim, they can fly in real life.