If arresting and prosecuting people for murder, rape, etc. had no discernable effect on those activities, led to as much corruption as the Drug War, and created all new problems we’ve never had to deal with before, then, yes, we should wonder whether they’re worth the trouble. But we never will, because those things you mention are malum in say, or evil in themselves, and people wouldn’t stand for perpetrators not to be brought to justice.
Drug consumption, production, and distribution are not evil in themselves. We criminalize them because we don’t like their direct and indirect consequences. If it’s a big waste of money, those consequences come about anyway, and new evils result from the direct and indirect consequences of the Drug War, rather than the drugs themselves, it’d be stupid not to considering dropping the whole charade.
Try telling that to the thousands upon thousands of drug addicted babies born to drug using parents. Try telling that to the children in schools who struggle with various learning and social deficits as a direct result of the illegal (and sometimes legal) drugs their parents used and abused. I see the evil in drug consumption on a daily basis.