The way I see it, if you do that, you're putting your life into the drivers hands (more so than normal). If he decides to suddenly stop, you keep going forward, all kinds of things can happen. Not only is there the "Physics" aspect to look at, but also the "Road Rage" aspect. I know that if a cyclist were to hold onto a car while I was piloting it, I would be pretty ticked off. Fortunately, I'm not the type who will get out and decide to bash someone's brains out for being dumb. Unfortunately, not everyone on this old world is as nice as I am. Some might just try and "teach you a lesson" by either accelerating hard, braking hard, or doing something else to try and get you into a wreck. Others might stop the car, get out, and give you a good beating (or shooting). People can be unpredictable. Don't risk it. Listen to officer friendly, because an annoyed driver won't always be friendly.
Edit: BTW, I applaud you for remembering the advice you recieved so long ago. Too many people would just block it out all together and not give such "juvenile" advice any thought.