I have been hit at an intersection by a car doing about 20, and to be honest, it wasn't bad at all. I mean, it hurt my hip, and I had deep bruising for a few weeks, but I walked away from it, and know better now. I have been right hooked before, which tacoed my rear wheel, and put me rolling onto the sidewalk at speed with a sprained wrist and a pretty messed up elbow. Both of these accidents (?) occurred while working as a courier/delivery. Somedays, I am almost glad that they happened.
I think that it does teach valuable lessons. If you get hit one good time when doing something stupid, you WILL remember never to do it again. I think that it has made me more comfortable in traffic, as I know that it would take a little more than a 20 mph tap to really kill you. It teaches you to be extremely aware of your surroundings. It teaches you to anticipate every messed up thing that motorists can do, and act as if they are going to do it, just in case. Remain as visible as possible, take the lane regardless of how much honking and pissed off shouting you get from motorists. Be predictable! Both of my accidents were completely avoidable.
If you get hit (and I have seen a few of my working friends in accidents), make sure you are wearing a helmet. The worst accident I have seen involved a kid not wearing a helmet, and he had bleeding ears. It was not pretty, but even he made it out okay. One only other guy I know who has been hit had some pretty serious injuries, but he is a mechanic at my LBS, and he seems okay.
Good luck out there!