When you get antibiotics from your doctor, he will tell you to consume the entire prescription. If you start to feel better half-way through, you don't get to stop, because there will be some stragglers left and they need to be killed off, otherwise their exposure to the drug could allow them to evolve resistance.

With spurious drugs, there are similar problems.

First, if the active ingredient is insufficient, then this is like the case of only taking some fraction of your prescription; you're just giving the bugs a chance to evolve in the presence of the antibiotic.

Second, even if 50-70% of the doses contain the active ingredient, you're playing Russian Roulette and in the end it's just like only finishing half of your prescription.

Third, there's a concept called herd immunity, where some critical percentage is immune to a bug and it acts to prevent the spread of infection. It's typically 80%+ for a population, so if only half of your antibiotics are legit, you don't get herd immunity.