Perhaps it still has a drawback.

This being a Potterverse it wouldn't be something straightforward like the way a condom insulates against body heat or decreases sensation. It'd be that the semen is magically transported into a nearby container. If you don't have a proper container prepared it ends up somewhere inconvenient like someone's pocket, or outer ear, or mouth. Or that both parties must spend a moment beforehand concentrating on a blue sphere, or the smell of vomit, or the sound of breaking celery. Or maybe it just makes a lady's feet numb.

So some people in some situations skip the contraceptive because they aren't prepared or don't want to deal with the complication.