Odd, you seem strongly against legalizing pot, even when the points you are making aren't in response to GP's. The question is not "are there harmful effects of smoking pot?", but "can we expect things to be better or worse if pot were legal, in contrast to things now?" Noting that I'm not the GP poster, allow me to rebut some of your statements:
If it was legalized it would be taxed, company profits would be attached, the FDA gets involved, legal protection problems for the use of it, like alcohol. In the end it might just be cheaper to get it illegally. In the end it might just be cheaper to get it illegally.
True, it might be more expensive, but some (many?) people will still prefer to buy it legally because of the FDA's involvement (quality control, secure knowledge it won't be laced with something else, etc), and of course because getting it legally wouldn't risk landing them in jail. It would also likely be more convenient. If it's more expensive and no one uses that option, what skin is it off your back, assuming you don't smoke pot yourself?
Have you ever seen someone high on pot drive? they are worse than drunks. A drunk serves because he doesn't have fine control of his body, someone on pot serves cause that's a pretty shade of yellow in the other lane.
Ok... so make it illegal to drive under the influence of pot. You know, the same way it's illegal to drive under the influence of alcohol, or the same way it's already illegal to drive under the influence of pot. Are you claiming that incidences of driving under the influence of pot would increase if it were made legal? (I ask because you never said that.) If so, please back up your claim.
All drugs react differently with different people(angry drunk vs mellow drunk is a great example) How do you know someone won't go homicidal while on pot?
How do you know someone won't go homicidal while smoking tobacco cigarettes, or drinking Red Bull (yes, caffeine is a drug)? Or while drunk? You yourself mention the variability in reaction to alcohol. The fact is, "homicidal" isn't a typical reaction to alcohol, nor to pot, and you know it. More importantly, even if that risk were significant, there are already people smoking pot! We're not talking about whether people should smoke pot or not -- we're talking about whether things would improve or worsen if pot were made legal. Pot use is already flourishing. How much would pot use increase if it were legal? Pot is so easy to get right now that I'd imagine most people who don't smoke pot now have other reasons besides its legality, and thus the increase wouldn't be terribly significant. There is evidence to support this idea.
So... you seem to be arguing just as much for making alcohol illegal as you are for keeping pot illegal. We've tried that before, and it didn't work out so well. Contrast that with the results of making pot legal, which has also been tried. From those links: one policy (alcohol prohibition in the US) "stimulated the proliferation of rampant underground, organized and widespread criminal activity.[4]", while the other (legalized cannabis in the Netherlands) has led to an average level of soft drug use, but with a level of "problem drug users (0.44%), well below the average (0.52%) of the same compared countries.[22] The reported number of deaths linked to the use of drugs in the Netherlands, as a proportion of the entire population, is together with Poland, France, Slovakia, Hungary and the Czech Republic the lowest of the EU.[23]".
With that supporting evidence, I propose that regardless of whether smoking pot is good or bad, harmful or not, etc, the effects of keeping it illegal are worse than what would result if we legalized it. The fact is, if people want to smoke pot, they can and do. And yet the government isn't getting any taxes from it, the FDA isn't ensuring you know what is and isn't in your pot, and the profits *are* supporting hard, violent criminals. The ball is in your court -- how would legalizing marijuana make things worse? (Please back up your claims.)