Imagine how much less of these dangerous synthetic drugs like spice and bath salts would be around if things like pot were legal. You wouldn't even have to ban it. Legalize pot, and watch the synthetic market dwindle down to nothing.
Would not dwindle, only increase. Check Norway where they tried to legalize drugs, use increased, crime increased to be able to buy the drugs. I don't know what the solution would be, but legalization is no the one.