First, lets understand what is meant by "Gateway drug". I never ran into a coke dealer or H dealer when buying booze. So, yes, Pot, when illegal can put people buying it near much worse people, opening that perverbial gateway. Second, when we lump pot in with all the other drugs, that makes many people treat the users of all those drugs the same. So some kid can't get a job at McDonalds because the owner is afraid the "Pot addict" will rob their till. The war on drugs has caused more harm by making kids, often bright, thoughtful, curious kids into outcasts. Funny how a teenage boy is many more times likely to commit a sexual assault when under the influence of alcohol than pot, yet many fathers would rather their daughter go out with a guy who drinks occasionally than one that smokes pot even less occasionally. 

As for the second question, the idea is that if we spent resources on education and treatment rather than enforcement, we would have less of both addicts and crime.