It's a must, especially with kids. One memory I have was my brother trying to get me into trouble because I rode into the water chest deep. My excuse, I was giving it a wash.
Last year I bought a cheap bike and went to thredbow down the proffessional downhill mountain bike course. Considering that the other riders had $1500+ bikes, It was very intense to say the least, It broke my bike on the second obsticle, and on several occasions nearly myself.