You don't need a dork disc if:

1: You carry a spare derailleiur hanger with you in the case you are out on a ride and you crash and bend the hanger on the bike, you can just replace the hanger instead of bending the hanger back and further weakening the metal on the bent hanger.
2: Learn how to keep your rear deraillieur adjusted and know how to compromise an adjustment when you crash and bend a hanger and you don't have a hanger.
3: Keep on top of checking the chain for wear. And to replace the chain when it's twisted and/or worn past a safe margin.
4: Carry a chain breaker, half link and a quicklink in your pack in case you do snap a chain and in result breaks the deraillieur hanger you had just put on or if you broke a hanger that isn't replaceable, so you can gheto ss your bike out of the woods.