The fact that its illeagle while nicotine + alchohol are legal is ridiculous. There are many reasons why its less dangerous than both of those, and in fact has been shown to be a beneficial drug in some cases, but most importantly it is non addictive meaning that people can stop and start whenever they want, it harms noone but them ( if it harms them at all ) so preventing them from doing it or even punishing them for doing it is wrong.
Also, even if you deny this, legalising the use of cannabis wouldnt actually increase the usage and would free up great amounts of police time and prison space, as well as reducing the use of harder drugs by removing the gateways to drug dealers, so no matter how dangerous you believe it to be, there is no reason not to legalise it.