Staz has a very valid point. just because a lot of people use majuana first it doesn't mean that it's a 'gateway' drug in that it encourages people to use other drugs, unless you also class alcohol etc in that.
yes this is anecdotal evidence (but it's so common-place that it seems anti-intuitive to completely disregard it), I know so many people who smoked a LOT of weed but by their own lights 'wouldn't touch chemical' (ie pills), and there's other people who yes, have tried weed, don't really like it heaps and don't do it regularly, but do other 'harder' drugs.
So yes, overwhelmingly people who take harder drugs probably have tried weed, and just as stas said, have probably 'tried' alcohol and cigarrettes too - becuase they're the sort of people who seek mind altering things
also, as for 'it alters the brain = bad', so does exercise. should we stop doing excercise because it releases endorphins and some people get addicted? should we stop drinking red bull and coffee? no. because they ARE relatively safe.
safe is most def a relative term.
living isn't safe. driving to work isn't safe. but it's relatively safe if you're not drunk etc.