If someone is an addiction prone individual there is no reason that marijuana would be more likely to push them towards other drugs than alcohol or cigarettes - both equally if not more addicting substances. The only difference is that two are legal and one is not - perhaps leading to, "Well, while I'm at it..." mentality.
I've never smoked anything in my life - cigarettes or marijuana - so I'm not just some pot head that wants it legal. I just think it is ridiculous that we keep sending people to jail for smoking some marijuana. The illegality also encourages crime and violence to spring up around the illegal selling of the substance - just like prohibition of alcohol.