You know, logically speaking, if marijuana is illegal, so should both alcohol and tobacco. Yet prohibition of those doesn't work, so we do have them legalized. I'm not personally a fan of it, but I don't see why it should be illegal. The rule for most things is the old Greek saying from Delphoi, 'nothing in excess.' Those things should be illegal whose faintest portion is an excess. I do not think that can be a proven case with marijuana, unlike some more potent drugs whose even trace use is perilous. And, of course, the situation is different person to person. There are also those to whom a slight bit of tobacco results in peril and addiction, and as such should for their own wellbeing stay away from it. We cannot, however, restrict such things in entirety, and some portion of responsability and judgement must lie on the individual person. After all, most anything in excess can become dangerous. Should we outlaw fatty foods because there are health problems associated with it in some people? Certainly not.
Now it might be argued that marijuana, unlike food, is not a necessity to life. This is certainly true, but if that is a tangental argument, then that regarding alcohol and tobacco is not. And those, even if they are not necessary, can add enjoyment, and add psychological benefits apart from the hard tangible health effects. After all, a pipe a day will hardly cause you health problems (national studies showed that even five a day did not cause discernable problems, and as such one or even two is not about to cause one peril.) Indeed, I'll argue it increases quality of life. I am very fond of smoking the occasional pipe... there is a great deal of relaxing and enjoyment involved, and it does not have a negative effect on my body... certainly far less a detriment than my abysmal sleep habits. In the end, I think I am in fact better off for smoking, so long as I stand by moderation. It is a distinct and cultured pleasure, in fact. And in some sense the rule that underlies 'nothing in excess' is this: that one must have control over one's self, rather than letting something exercise the control. If one can enjoy tobacco or alcohol, or even marijuana, without the compulsion and with discernment and will, not subverting the will to the impelling force of a 'need', then it is in my view ethically right. But if one is set subservient to a desire - and this is in everything, whether it be substances or even moods, such as love - then this is wrong. Things should be enjoyed, but to be truly enjoyed one must be able to take it up and place it down at will, and not at the behest of an overriding compulsion.
Therefore the same enjoyment cannot be said for everyone, in particular those who have a natural disposition to addiction in a certain form. That has been proven now, that a certain gene makes one more likely to be addicted to tobacco, and to get cancer from it. But all the same, for those who can practise moderation, there is no ethical reason why it should be wrong. I think the same could be said for marijuana. I do not favour it because it dulls the mind, rather than sharpens it (for that reason I prefer my pipe/cigars and coffee), but I cannot see what is ethically wrong in moderation.