Good UX demands the correct distribution of visual weight. If a design requires many scrollbars, and those scrollbars have static styles, than those scrollbars dictate the visual weight of the rest of the app. This can make it impossible to design a nice intuitive UI around. So vital can definitely be an appropriate description for certain UIs

Normal width scrollbars use way too much space. This is a professional app with a complex dense UI, similar to ones used by image editors, music production software, 3d modelers. All of these use non-native controls, since native ones are too "wasteful"