Against: Time to put, take off This depends on your point of view.
Against: Inconvenient to carry when off-bike.
Against: Does nothing to stop brain detaching from skull membrane which is the most important damage to avoid (cuts are mere flesh wounds)
Against: False sense of security/moral superiority If polystrene was that good then they would make lorries and formula one cars out of the stuff. Look at a big lorry coming towards you and now imagine that a cheap bit of Chinese polystyrene is going to give you Superman-esque powers of invincibility.
Against: The Dutch and Danes do not wear them, although they do buy them for children they cycle with. Although road conditions may be different in Copenhagen, they still have fast moving automobiles.
Against: Most helmets do not fit properly I have sold several hundred helmets in my time, including more than a hundred on one day (at a trade show) hence I have experience of getting them to fit, which they can be made to do. However, most helmets that I see worn are not adjusted correctly and with one's little finger you can typically flip the lid back from being on the top of the head. With a bit more force, e.g. from the rider's mass decelerating from twenty miles an hour, the ill-fitting helmet could harm the rider, strangling them. This is particularly the case with children's helmets.
Against: Reassures mother She grew up when the car was king, she continues to trash the planet with her automobile, she has no intention of ever riding a bike again, she completely believes the FUD about helmets and has the occasional road mishap herself - sound familiar? Of course you can do as you are told - by her - after all, mum knows best, doesn't she? She actually has what psychologists call 'projection' and 'denial' on the go. There comes a time when you should tell her to take a hike, or at least a bike.
Against: Anecdotal rubbish by the helmet trusters We have all heard about so-and-so's little Johnny that would have had his head split open had he not been wearing a helmet, haven't we? You cannot argue against people and their anecdotal evidence, even if little Johnny was riding back from the pub pissed on unfamiliar roads having not been much of a cyclist in the first place. If you have not hit so much as a kerb in 20+ years then you can still be deemed irresponsible by the helmet trusters.
Against: Poor quality helmet product. Only the MET helmets have straps that are soft and lie flat on the side of one's face. Giro and Bell helmets have straps that want to dig in at a 90 degree angle, typically coupled with a buckle that can painfully pinch the skin.
For: Keeps head warm in winter a helmet can keep your head out of the cold and rain.
For: Great for off-road cycling off road with any type of bravado is a bit silly without a lid.
For: Professional road racing You need a lid to road race. Unfortunately for Wouter Weylandt in this year's Giro d'Italia his helmet did not prevent his skull cracking after a crash and he is with us no more. However, no lid, no race.