Many would. There are different types of people. The first group uses drugs despite their illegality. The second fears the consequences of getting caught doing something illegal and refrain from doing drugs because of that. The third group wouldn't do drugs even if they were legal.
It's the folks in that second group that behave differently if drugs were legalized. Many who refrained before would start experimenting. And some of those experimenters would end up hooked, much to their, and society's, detriment.