Sometimes conventional wisdom isn't so wise.

Trailer tires are not helmets nor do they serve the same purpose. Tires are left outside exposed to the elements all year - why would anyone do that to a helmet. It's akin to never lubing the chain and wondering why it needs to be replaced so often. Parking your bike in the rain and wondering why everything gets so rusty. Take care of your kit?

For the average rider there's nothing miraculous about the foam in your bicycle helmet that limits its effectiveness to a couple of years. The design is simply to put something between the road and your soft noggin in low impact crashes; for most recreational riders about 30km/h max. More likely, falling over when one forgets to unclip. The helmet allows the foam to go crack instead of the skull. If you get centerpunched at 60mph no helmet will save you - the head may be intact but it will be depressed inside the thorax.

You could make an effective helmet out of 3-4 inches of compressed felt inside a shell if you wanted, but it would be cumbersome, heavy and hot. Most helmet manufacturers are really just reducing those common drawbacks by using thin, perforated foam instead. Moderately priced helmets hit the mark but people want lighter and sleeker so the compromise comes in durability.

They are a lot like construction hard hats. You don't need space age materials or to replace them every couple of years. They just keep tools from doing severe damage when dropped off the scaffolding. But if you want a fancy lightweight one it will probably be more susceptable to damage and need replacing. If you do buy the fancy one, probably leaving it to bake in the car for several years is not a good idea, as is never lubing the chain.