I believe derailing is a natural part but should only be banned where it's against the will of the OP. Someone who makes joke threads or ones where the topic is blurry will and usually expects/wants derailment. They and the site overall 'gains' from the derailment as it adds content and lengthens the thread
A serious or defined topic however only be hurt by derailment and that's where most actions against it are taken.
Just my two cents