In practice, only a small percentage of people are able to recline enough to benefit from a headrest. Most people, who sit mostly upright, only need lumbar and upper back support. In their case, a well designed backrest is sufficient.
Adjustable headrests offer the most help to taller people or people who recline or wish to use the headrest to stabilize the neck more. An adjustable chair allows for optimal customization with a removable or height-adjustable headrest.