This to me is something that stems from something like an old-wives tale. Like you should wait 30 minutes after eating before going swimming. It's not based anywhere in science and it's more or less a method of deterring drugs; similar to the entirely failed D.A.R.E. program. It's ignorant to think that it's a steadfast rule that once you try pot, BOOM, "LET'S GO DO BLOW AND FUCK HOOKERS!". Right...certain people will want to do that. Those people will have had that mindset from before they smoked weed; the marijuana itself does not do that. I myself haven't smoked in a year (after being a habitual smoker for years), it just kind of tapered off as my priorities changed and I grew up a little. But once I stopped smoking, I didn't feel the urge to try anything else because I know my limitations and I know I have a very addictive personality. As a matter of fact, I had tried other drugs before I even tried weed; once I started smoking, it had the exact opposite effect...I didn't want to do anything else. As almost everyone else has said, it is completely and utterly dependent on the user.
Sure, some naive pimple-faced kid might feel the urge to use another drug once he's gotten high, perhaps even in the middle of it, but to think that once you smoke pot, one's mind automatically wants to try different drugs is absolutely ridiculous. This is not a blanket rule, it's a jaded, dusty-old way of thinking that is quite frankly completely out of the realm of reality; especially these days when drugs are more out in the light and people speak more frankly about them instead of acting like they're Satan's little helpers as it was in years past.