I think that what parents fail to realise is that 'bad' experiences in life teach us just as much about the world as the 'good' experiences. You can't see things in purely black/white. Accidents, mistakes, and injuries (emotional or physical) all tend to teach us SOMETHING about the world around us.
The pains that I've experienced in my own life have taught me to have empathy for others who experience the same pain - and it allows me to help them through it, because I understand what they're going through.
In the same way, mistakes teach you how -not- to do something. I'm bad with directions, but when I get lost somewhere and eventually manage to find my way back...I remember.
Parents don't realise that keeping their kids from these bumps and stumbling blocks makes them much less prepared for life. If you've never struggled to achieve something, how will you know how to do it in the future - when no one's there to hold your hand?
Life is like a mountain, where even the stumbling blocks get you closer to the peak.