While this is a good rule of thumb, accidents happen. Sometimes, even if you're riding within your limits, something happens and you want your helmet to be there for you.
Also, I firmly believe in progression. It's what I like most about mountain biking: trying things that are outside my comfort zone. that's why I have a dirt jumper and go the skate park. of course, you have to progress slowly, but that still requires an element of risk.
I've cracked a few helmets in the past couple of decades riding bikes. I'm happy I have them, and I'm always paying attention to new technological improvements. of course, I'd be happier knowing those improvements have a basis in science, but as long as they don't take away from the baseline safety of the helmet (i.e. meet the same basic safety requirements and pass the same tests), I'll gladly shell out for a helmet claiming to have even more safety built-in (though an independant test that proves these claims sure would be nice).