there are people who will not actively seek out illegal drugs. if, however, several highly addictive drugs are suddenly legalized and become as easily available as alcohol or cigarettes we will see a lot more non-addicts and non-users casually testing them out. so what's the problem there? it has to do with addictiveness and the movement from casually testing drugs to serious addiction. if the distance between those two states can be measured in one sitting then i think there is good cause to make illegal their sale and use or if not illegal then at least severely regulate them.
i think the situation most people are afraid of when it comes to legalizing certain drugs is that they will then become fodder for the next tabacco-like industry... which is to say nearly ubiquitous... if not in use then certainly in availability and visibility.
tabacco/nicotine actually fits into the category of drugs that are just downright stupid of people to use. although its so entrenched now that to remove it all at once by force would cause so much disruption and uproar that the costs would outweight the benefits. instead the federal government, state governments, cities and local establishments have been slowly nudging the tabacco industry closer to the exit door (literally and figuratively).