However, you did touch on something else that I think is important: the idea that marijuana is a gateway drug to harder and more dangerous drugs. To a certain degree, I think this is true: for many people, buying marijuana is their first introduction into the underground economy of drugs: if the guy who sells you pot can't sell you harder stuff as well, he probably knows someone who can.
Effectively, what we've done through our legal structure is create a ladder for people to climb up, where every higher rung is a nice, easy step from the one below: first, you start with legal drugs. Then, you move on to illegal "soft" drugs like pot, that have equivalent (or lesser) harm than legal ones. This makes the step from "soft" to "hard" drugs much more manageable.
Personally, I do have a problem with hard drugs: I think that they can be very damaging, both to individuals and to societies. I would be quite happy if we could eradicate drugs like crack and heroin. I think the best way to do this is to make that climb up the "drug ladder" as difficult as possible, and I think the best way to do that is to take out a "rung" by making pot legal. If marijuana users never have to delve into that illegal drug market, then the step from pot to harder drugs become much more daunting, and therefore one that fewer people will take.
And this doesn't even address the significant harm associated with the illegal trade in drugs. Around here, biker gangs have made the production, distribution and sale of marijuana big business... and they have no qualms about using violence to gain market share from their competitors, much like the mobsters and bootleggers who engaged in violence for the alcohol trade during prohibition. And just like with alcohol, if marijuana is made legal, this violence would go away.