That used to be true, but now they've come out with helmets that have better side impact. The only one of these that looks attractive to me is ~$200, so I have yet to pick one of these up, but one for my girlfriend is on the short list as her helmet took an impact recently. It doesn't show any real damage, but I suspect it isn't as effective as it was before. Besides, she's had it for a decade, and chances are the time and sun have probably degraded the materials some as well.

Luckily I have yet to crack a helmet from impact, I have retired two due to the pads being shot, the straps being worn out, and the clicky thing at the back that adjusts the fit being broken.