Answer: The vestibular system is useful in spaceflight for maintaining visual fixation. Without a vestibular system, vision would be degraded when the head is moving in relation to the object of regard.
The eyes move with the head. Vision is blurred during head movement unless there are compensatory eye movements. The vestibular system's job is to drive those compensatory eye movements.
The vestibular system senses rotational movements of the head and compensates for them. The output from the vestibular system connects with the oculomotor nerve (the nerve which controls the majority of eye movements). This is called the Vestibulo-ocular Reflex (VOR). It is responsible for maintaining visual fixation during head movements.
To illustrate the effect of VOR, hold a finger in front of your face and wiggle it side-to-side rapidly. Your eyes are not able to track the finger’s movement, so it is seen as a blur. Now hold your finger still, but rotate your head rapidly side to side while fixating on your finger. Fixation is much better (the image is much clearer) than when wiggling your finger.
If both finger wiggling and head shaking are each done at the same frequency (count out loud as you do each), it is clear how much better fixation is with VOR during the head shake.
In a space mission, the VOR will assist in maintaining visual fixation during both voluntary head movements and involuntary head movement i.e.: shaking of the spacecraft.
It would be interesting to test adult subjects with compromised vestibular systems. The subjects described in the OP would not be suitable for this experiment since they all lost their vestibular function in childhood. Developing brains have a greater capacity for plasticity than adult brains. This plasticity would confound the results.