Drug dealers would lose most or all of their business, which obviously would lessen crimes. If marijuana was legal, it wouldn't be handled by dealers, which means the chances of it being laced with a more harmful drug, would be much less.
Smoking cigarettes are more harmful than marijuana, because of the use of tobacco.
There is always the problem with marijuana being the "gate way drug" and it may be true for some people. But people were, are and will be smoking marijuana, making it legal will just lessen crimes and problems.