The EntityDismountEvent should implement the Cancellable interface so plugins can cancel the event.

This feature would be useful for making mobs like Dolphins ridable since you get dismounted from mobs when in water.

Right now the only workaround is re-attaching the passenger, but results in a glitchy experience.

With a cancellable event the rider would never get dismounted for a much better user experience without all the visual glitchiness.