I can only speak of my personal experience and the people I know, but I've never met anybody who got into the "hard" drugs because they wanted stronger bones. Everybody I've met who got into heroin, meth, crack, etc (about a dozen people that I know well enough to know how they got started) has been in it for the high. They all started with marijuana and alcohol, as they're the most readily available and socially acceptable recreational drugs. I'll keep my eyes peeled for any junkies who are in it because they believe heroin is a good source of calcium, though.
Will legalizing marijuana produce a a whole bunch of new hardcore drug users? I'd say it's probably unlikely. It's already readily available and the people likely to abuse drugs and become hardcore addicts are most likely already smoking it, unless they've moved on to something else.
My personal position is that I'm 100% against the legalization of the "hard" drugs, but I can care less about marijuana either way. I don't believe that potheads are a threat to society, however I am firmly convinced that crackheads, junkies and tweakers are.