A number of injuries have been attributed to the practice of head-banging to music.
The following list are individual case-studies, so should be considered little more than well-considered anecdotes.
Carotid Dissection - a 15 year-old died of an aneurysm.
Subdural Haemorrhage - 29-year-old had headaches for a week.
Fatal Subdural haemorrhage - No abstract available.
Mediastinal Emphysema - A 34-year-old guitarist regularly expeirneced neck and chest pain after concerts.
Neck weakness and long-term neck pain and upper limb parasthesia - a 16-year-old suffered a neck injury while head-banging that continued to show symptoms for 4 years. No validated diagnosis was made.
Vetebral Artery Aneurysm - a 15-year-old drummer suffered an aneurysm from violent head and neck motion. "He is neurologically normal 1 year after surgery"
Basal Artery Thrombosis - 20 year old musician. While it is likely that head-banging caused the problem, it is not clear. He suffered further in hospital during treatment (I couldn't quite understand the details, but "the therapeutic intervention had resulted in significant neurological disability.")
This article goes on to provide several references for head-banging related injuries (mostly covered elsewhere here).
The following studies are more than just case-studies.
Whiplash - Thirty-seven 13/14 year olds partook in a charity dance marathon for 7 hours. 82% of the girls and 17% of the boys who partook in headbanging during three heavy metal songs suffered cervical spine pain for 1-3 days. 26.2% of the girls and none of the boys who did not partake in headbanging suffered cervical spine pain.
Head and Neck Injury - This was a biomechanical analysis of head-banging, rather than measuring actual outcomes.
An average head banging song has a tempo of about 146 beats per minute, which is predicted to cause mild head injury when the range of motion is greater than 75°. At higher tempos and greater ranges of motion there is a risk of neck injury.
To minimise the risk of head and neck injury, head bangers should decrease their range of head and neck motion, head bang to slower tempo songs by replacing heavy metal with adult oriented rock, only head bang to every second beat, or use personal protective equipment.
Clearly, head-banging has been associated with injuries, including some fatal. The brain injuries appear to have had immediate effect.
I found no evidence that long-term head-banging causes cumulative brain-damage, even in an article that attempted to summarise the risks. The biomechanical analysis did not warn against long-term use, but merely advised on prevention techniques.
Nonetheless, these immediate risks should be considered when deciding whether head-banging is worth it.