A lot of helmets are designed for one time absorption. I know when I was snowboarding a lot, some of my friends would get a new helmet every season because they may have hit an icy patch during a fall without realizing it. Once it is compromised even sightly it won't protect you from the big hit.
I certainly had enough bad falls on the terrain park that I'd keep a spare helmet in the car.
A lot of helmets are designed for one time absorption. I know when I was snowboarding a lot, some of my friends would get a new helmet every season because they may have hit an icy patch during a fall without realizing it. Once it is compromised even sightly it won't protect you from the big hit.
I certainly had enough bad falls on the terrain park that I'd keep a spare helmet in the car.
We have to get new hardhats every so many years, they claim there no good after that. I think it's also said about bicycle helmets. Maybe it's just to sell more hats.