The bed of a pick-up truck is for CARGO, not people. If you care about your passengers (or your pets), do not allow them to ride in the back of your truck. You may be the best driver in the world -- but you are not always totally alert while you are driving. You never know when some other driver may cut you off or cause a collision some other way. Don't put people or animals at risk by allowing them to ride in unprotected places.
In a collision, the sudden deceleration or other crash forces will launch the person out of the bed. Most of the time, no amount of "holding on" can prevent this (even at slower speeds). In some collisions I have attended, pick-up bed passengers thrown out this way have been subsequently run over by the vehicle -- or other vehicles.
http://www.roadtripamerica.com/DefensiveDriving/Rule65.htm