re: Give me one good reason why marijuana should be illegal while alcohol shouldn't

I'll have to both agree and disagree with you there. In both cases, tolerance does develop and the user does need to use more in order to get the desired (subjective) effect, and the person who has used longer probably will seem to "have it together" more so than the naive person's first time drunk or high. There's no arguing that.

Some of the physical effects (slowed mentation, reaction time, etc) are consistent with drug levels regardless of whether the person using is naive or a chronic abuser. If you read the review article I linked, it actually specifically mentions that marijuana users do experience cognitive decline that is measurable but not noticed by the abuser. I don't believe reaction time was measured, but the other tests were all related to the concept of concentration. It's extrapolating a small amount with no hard numbers or study (not to say that one isn't out there, but it's not mentioned in the one that I linked), but I think it's at least reasonable to assume there is going to be a delay in reaction time (as there is with all "downer" substances that are abused. In everything I've read to date, despite memory/recall, ability to perform specific tasks not being challenged after chronic use, reaction time usually still lags. How significant is another question, but the concept of "unnoticed effects" is the point I am trying to make) Because of this, it's probably very likely that even the chronic user has delayed reaction times and cognitive functions when taking in less, even if it's not realised by him. As to what that amount is or what level of concentrating THC is responsible for such decline, I don't think there is so much as a good estimate at this point. You can bet the house on a substantial amount of research going to find that number and how to measure it somewhere in the peri-legalization window.