Cycle helmets are light and made full of holes because when using a pedal cycle you generate heat. They do NOT offer the same level of protection as say, a motorcycle helmet - they are not designed to. They can offer limited protection - but it is *limited* to minor knocks. If you drop one *once* you are supposed to replace it. That should give you some indication as to how robust (or not) they are designed to be.
Cycling is a relatively safe activity in the great scheme of things. Millions manage to do it without serious harm or death being the result of cycling. Indeed there are many health benefits to be gained from cycling.
If you wish to wear a lid, by all means do so, but do not think it will save your skull if you in the highly unlikely position of going under the wheels of a bus. It is not designed to do that. It may, however, save you from being nagged by your nearest & dearest and it may offer some protection in a minor scrape.
I wear a lid, it forms a nice mount for a LED head torch which I have found to be one of the most useful things I have come across when cycling at night. In winter, it keeps my head warm. The rest of the time it sort of keeps my hair in check and in the event of a spill where I require hospitalisation, it will stop the stupid question of "why weren't you wearing a helmet?" when it's my leg and shoulder that is injured... Luckily enough, it does not overcook my head when cycling in summer.
IIRC there was a long thread on uk.rec.cycling where the figures regarding cyclist deaths/head injuries being the cause were examined. IIRC in the vast majority of cases, the fatal injuries were mostly a combination of major trauma to various parts of the body, rather than head injury.
The best thing a cyclist can do to minimise their risk of ending up hurt is to learn to cycle safely and assertively in traffic. The wearing of a lid may or may not be part of your additional cyclist's toolkit of staying safe along with wearing gloves, using lights, keeping your bike well maintained etc.
I would highly recommend to any new/returning cyclist that he or she get themselves some cycle training to the national cycle training standards as these have been specifically designed to help cyclists acquire the skills and confidence to cycle as safely as possible on today's roads, by cycling assertively (not aggressively).
Hope this helps.