Karma biting me in the ass would be me making the document and not using it.
Plan for the worst. I recently helped a rider who slid under a truck after a "minor" highside. If he would have been knocked out from head trama I would not have been able to contact his wife and sister so they could be with him ASAP!
A rider went down on Palomar Mountain a few weeks ago and his wife didn't find out until 5pm because no-one knew her number and the guy's phone went up in flames with his bike. He was in a coma for days so he could not provide any information on his medical history. His wife was at home crying her eyes out in despair because she felt something bad had happen since her husband was so late getting home.
One sheet of paper might be something that saves your life (if you have medical allergies or a health condition) or it might save a loved one hours of grief when they can't find you because your all balled up in a hospital or morgue.
Damn, now I'm babbling on and one. :D