While serious head injuries involving concussion and/or memory loss are not as depressingly common in rugby as they are in American football, the possibility of head injury does still exist: if you practice correct tackling form on an opponent, for example, your head will end up right by your opponent’s knees. Again, rugby is a collision sport, and the probability of getting bonked on the head from time to time in the normal course of play is high.
Most rugby players are willing to live with that risk… until their first serious concussion. Take it from someone who knows: there’s nothing like a good spell of amnesia to make you re-consider your position on headgear.
An additional benefit of headgear for locks and – to a lesser extent – number 8s is the prevention of cauliflower ear that can be an occupational hazard. Rugby shorts have to be made of tough material, and players at these positions can spend a lot of time at practice and in matches in scrums with their heads squeezed up against said shorts, repeatedly having their ears abraded unless they take precautions. In the days before helmets, players at these positions would typically wrap medical or electrical tape around their heads so that their ears would be covered.
The modern headgear has several advantages over this bit of battlefield medicine, not least of which are ear-holds that let sweat out and sound in. The part about “sweat” should not be discounted; even though the headgear is made out of relatively light, breathable material… well, you’re still wearing something on your head that doesn’t necessarily need to be there. The temptation in hot weather then becomes to leave the headgear off and take the risk, especially if one happens to be playing sevens, where the risk of head injury is substantially lower.
Unlike mouthguards, though, there is not as much price flexibility with headgear; a decent one will cost from $50US to $90US, and anything cheaper than that might not be worth wearing, except as a strange-looking party hat.