I'm not exactly sure of where I stand on the decriminalization issue, but as for legalisation I say no.
1. Compairing it to tobacco is not a valid argument. This is because tobacco is known to be harmful and slowly laws are becoming stricter regarding the use of tobacco. Also, tobacco does not alter the physical state of mind other than providing relief from the addiction. ie getting high will raise the chance of you getting into a car accident but tobacco will not.
2. Tobacco may be more addictive than marry jane, but chronic is an addictive substance as well, and it doesn't do much to say, well, it's not AS addictive as this other harmful legal substance. And it's been proven to be just as addictive, if not more so than alcohol.
3. Alcohol is damaging to the brain, but that doesn't mean it should be banned. When someone smokes weed, it's for one purpose: to get high. Alcohol on the other hand, for me, my friends, and family at least, drinking is just wine at dinner, a bear now and then or a couple shots of vodka here and there. This means that: we're not getting drunk, therefor we are not damaging our brains any noticable ammount, and therefor it is not an addiction. That's why weed should not be legalised under the reason that, 'Alcohol's legal, so lets legalise weed.'
4. Weed is just as harmful when smoked on a consistent basis as tobacco is. Because when smoking weed almost everybody holds it in their lungs, it leaves a lot of time for the body to not only take in the THC, but also the harmful chemicals, such as tar (causes fatigue and lung damage/cancer, destroys oxygen receptors in the lungs and lowers the body's oxygen supply over the long term) and carbon monoxide (mild brain damage in small ammounts, in larger causes loss of conciousness or death, works by binding to the receptors in the blood stream which normally bind to oxygen). Yes, tobacco does all these things, but weed does them too. It has been estimated that depending on the size of the joint, one joint can cause the same damage as 4 cigarettes to one whole carton.
5. If it was legal, more people would try it, and some of those would become addicted and suffer greater damages then if they would become addicted to cigarettes. At least if it's illigal, those that become addicted did so by going through bigger ethical questions. In other words, they got what was coming to them.
I also conclude that weed should be decriminallised, as people shouldn't have a criminal record for doing something stupid.