It seems to me that there are actually two different conversations going on here. One is the simple one about whether Marijuana should be legal. First off, I don't smoke it, or any other drugs, and very rarely drink (six pack lasts me a month or so). Up until my early twenties I did very large amoutns of just about anything that was around whether injested or smoked. Nowadays I don't have time for the crap. It does dull your senses and impairs your judgement (as does alcohol, with the addition of killing all of those wonderful grey cells). Although I have a dim view of the drugs and quite a few people who participate in their use, I think that they should be legalized (not just marijuana). Prohibition didn't work with alcohol and in the long run won't work for anything else. It just creates a multi-billion dollar market that never gets taxed and operates under the radar of the goverment (for the most part). The war on drugs is a joke and a waste of my paycheck. I think that what you do in the privacy of your own home is nobodys business, especially the government.
On the issue of driving under the influence, or participating in other activities that might harm others, the penalites cannot be too harsh. Anyone that gets behind the wheel of a car, or other equipment without their full and complete attention is a friggin idiot and should do time (at the minimum).
I also think that even though we should have the right to do whatever the hell we want in the privacy of our own homes, our employers ALSO have the right to drug test if they should choose to do so. If you don't like your employers policies, work elsewhere.