As Edward Snowden once argued in an AMA on Reddit, a zero crime rate is undesirable for democratic society because it very likely implies that it's impossible to evade law enforcement. The latter, however, means that people won't be able to do much if the laws ever become tyrannic, e.g. due to a change in power. In other words, in a well-functioning democratic society it must always be possible (in principle) to commit a crime and get away.
Take some examples of laws that have changed over time. Say, interracial marriage. It was illegal in many places to marry someone of a different race. If this had been perfectly enforced, no one would have ever dated or see couples of different races, and people would have had a lot harder of a time exploring and realizing that the law was wrong.
The same thing could be said about marijuana legalization. If enforcement was perfect, no one would have ever tried marijuana, and there would have never been a movement to legalize by people who used it and decided it was not something that should be banned.
We need to be able to push boundaries so they can move when needed.
The problem with perfect enforcement is it requires the same kind of forethought as waterfall development. You rigidly design the specification (law) at the start, then persist with it without deviation from the original plan (at least for a long time). In your example, the lawmakers may still pass the law because they don't think of their kids as drug users, and are distracted by some outrage in some other area.
Giving the former discretion was a way to sneakily contain the worst excesses of the latter.
Alas, self-interest isn't really something voters seem to really take into account.
Eastern Europe went through a similar transition. Before the iron curtain fell, the eastern bloc operated on favors more than it operated on money. This definitely isn't the case any more.