To the myth that marijuana is a gateway drug (adding on to what was already said)...as Chris Rock said, drugs sell themselves. Also, in the Netherlands, there is far less hard drug use than other countries where marijuana use is less prevalent.
As for legalization of it, I don't smoke but I'm all for legalization both for economic AND social reasons.
Economic reasons are obvious, the government COULD tax it and profit from it. Sure, people will argue because of how easy it is to grow and the fact that the police don't want to lose their funding contribute to the idea that legalization and taxation would not be possible but this does not seem to be true. To allow police funding to continue, just set up a system where possessing large amounts, without a permit of some kind or growing more than a certain amount would be illegal. Thus, police could still get their money to prowl around for that (and for hard drugs of course) and the government would be able to tax whichever company (probably tobacco companies, as they already have the infrastructure). Of course, this is just a rough idea I've been kicking around in my head, but it seems that it would work.
Social reasons may not be as obvious but are definitely a more deciding factor for me (and are tied in to other economic reasons..everything relates back to money). Many people go to prison for possessing marijuana, they get FELONIES for possessing marijuana (remember, a felony conviction in most states means you lose your right to VOTE, so essentially you could become a lower class citizen in the eyes of the government simply because you had a bunch of weed). In some states, there have been prisoners convicted of MURDER who have received parole before ones who merely sold weed (I read it in the book "Reefer Madness" (some dude heavily researched this stuff, and a couple of other issues and he had his own sources which I checked out then) and that was a while ago so I don't recall specifically where but feel free to correct me if you find otherwise). Over 10 million people get arrested in the United States for marijuana related offenses and as I mentioned, many of these receive felony convictions (about 250,000). All this for a drug less harmful than both alcohol and tobacco. In addition, in prison, people frequently learn, in a sense, more ways to commit illegal acts and many end up doing so. These prisoners also are a drain on our society, prisons are NOT cheap places to run.