Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

dan i occasionally play cricket, i use a box when i bat, and you're right , it's probably a good idea to wear it on my way to the ground... just in case :p)

Just in case someone bowls a ball at your genitals whilst cycling to the ground?

As that is the thing which it is designed to protect against, whereas Dan's point is that if you are cycling with your helmet attached to your bag you are not taking advantage of it.

Therefore, using your cricket box analogy, you would have it clipped to your belt in a jaunty fashion for some of the match, as your ability on the cricket field is obviously much better than most cricketers, especially "nodder" cricketers, so you don't really need to wear it, certainly not all the time.