You can not overdose on THC. Technically you can but it has never happened. Theoretically you would have to smoke like well over your body weight in pot in a small amount of time for it to kill you. Of course, table salt can kill you if you eat enough of it at once as well.
Alcohol will destroy your liver much quicker than weed will your lungs, it is much more addictive, it causes people to act violently, and it impares judgement much worse in most cases. I expect a drunk person to be dumb enough to drive a lot more than someone high. When people are drunk they think they can do anything, its really the opposite with weed, you usually feel less like taking that chance. You can tell when drunks are on the road because the speed, swerve and take chances. Usually I can also tell when someone high is on the road because they are going about 15 mph under the speed limit and slow really early before a light. In Amsterdam they say that the only problem they have really is with alcohol, not the heroin junkies, not the potheads, but the drunks.
Also, medical marijuana is exactly the same as street weed. There are different forms of it but its all THC. Scientists have worked with it a lot to try and dull down the side effects while maintaining the pain killing effect but of course none of this has been properly tested since its illegal. Medical marijuana has many uses other than just pain killing too, its been used in pill form(synthetic THC) to help control autism, parkinsons, arthritis and more. Like you said google it you'll learn a lot. Furthermore it has no long term side effects unless smoken, unlike most Opium based pain killers on the market now like Hydro/Oxycodone, Codeine, Oxycoton, etc. These are rough on the liver, addictive, easily tolerated after a few weeks of taking them, can be overdosed on and are often prescribed for very long periods of time, often to elderly with chronic pain. Ederly men and women would most likely benefit most from medical THC as it could greatly reduce health risks involved in most Rx pain killers, but I'd be extremely interested to see how it could be used to help autistic children. The fact that a natural drug like THC can be outlawed for monetary reasons(no matter what you say) when it has so much potential is really just wrong.
There's another aspect of this however. The drug war. Its expensive and extremely inefective and leaves many good people dead or incarcerated. I don't feel like going into the drug war thing right now but I guarantee it carries much heavier prices than legalization ever would.