The anti-drug campaigns tell us that marijuana is a gateway to other drugs. This argument sometimes even takes the form of a slippery slope fallacy, asserting that marijuana consumption inevitably leads to consumption of other, "harder" drugs. Plenty of people will argue that the majority of marijuana users will stick to marijuana, never using other drugs, but there's an effective way to reduce the chances of drug users ever having such a gateway.
The main reason marijuana users have access to other drugs now involves the nature of the drug trade. Dealers will sell what makes them profit (not all, but many), so they will offer other drugs, such as LSD, ecstasy, heroin, cocaine, or meth. As such, those who are willing to experiment with other drugs will have the access to them. If legalized, marijuana will be sold in state stores, dispensaries, and possibly grown at home, depending on how the laws for it are written. Legal outlets will not risk their business by offering drugs that remain illegal. Also, the chances that marijuana will be laced with other drugs (it is sometimes laced with cocaine or formaldehyde) become vastly reduced.
Legalizing marijuana for recreational purposes, as Colorado and Washington have done, creates a revenue stream for the state, but it also stops the practice of making criminals out of non-violent drug users. Furthermore, it takes away an income source for opportunistic criminal enterprises.
It would be interesting to study how much reduction in use of harder drugs takes place in Colorado and Washington over time.