There's nothing worse than a horror movie that isn't scary. When the entire purpose of your movie is to scare people and you fail to do that, then it's hard to deny that the movie completely failed at its job. There can be a lot of reason why a horror movie isn't scary. If the actual concept isn't terrifying, then it's going to be an uphill battle to get audiences into a state of fear and panic. If you're making a movie that relies on special effects, you better be sure that they're up to snuff or else your movie is going to be mocked and ridiculed (see The Haunting). What usually happens though is that a director will take the easy way out of scaring its audience; they'll resort to jump scares.
We've all had our experiences with jump scares, whether we like it or not. They're those moments where something will pop out and yell "BOO" to get a cheap shock moment out of the viewer, or give them a false scare by having a loud sound effect chime in to startle you. Either way you slice it, the majority of bad horror movies tend to resort to jump scares as their main tricks and even major horror movies tend to use them every now and then.
For most people, the very idea of a jump scare is enough to invalidate a movie based solely on principle. If horror movies like The Babadookand The Witchcan create a horrifying atmosphere without ever resorting to jump scares, then the very idea of a jump scare is a useless tool. It's a tool that doesn't need to be used because you can create effective horror without ever needing to use it. I would like to argue that not only do jump scares have a place in horror movies, but they can create some of the best scares in any given horror movie.