There are a number of reasons why neutral alignment is sought for spinal immobilization. One would be extrication from a challenging location. The patient must be moved to be placed on the back-board, and grossly angulated anatomies create a great challenge.
Neutral alignment facilitates airway management.
If the patient's condition were to change during the transfer, the driver would have to stop and climb in back to assist in repositioning to achieve airway management. I think it's better for the patient to anticipate difficulty and be prepared to manage with the least possible delay. Vertebrae are seldom obviously broken. Certainly the number of patients immobilized is greater than the number of cervical fractures. If you work in a hospital, the presentation of extrication skills and the environments patients come out of may be a real eye-opener.