As far as a helmet for a larger noggin' goes - Airframe this is the best solution I found, short of going for a real lid.
@Eddie The Headshot - I wear one for protection, due to medical reasons. I suspect the majority of people wear them for the "tacticool" (I loathe this term) look. @Seven more or less summed it up nicely.
On a sidenote at sites which allow TAG rounds and events which allow them I personally think they should be a mandatory requirement.
@ImTriggerHappy - As Seven has stated, some is better than none, for me personally I play a lot of CQC and it's more the danger of taking catching my head on a low crawlspace/cut through. I realise that if I want any serious protection I'll need something actually rated for it but for just catching a door frame I've found my current one to be more than sufficient for the task.