>it isn't exposed to browsers
well, I have a strong position that it should not be exposed to browser. Exposure of system settings/preferences should be minimized as this provides additional user-profiling opportunities.
Exposed to the browser doesn't have to mean it's exposed to websites. The browser has access to all sorts of system settings that websites aren't allowed to view. Although admittedly it's hard to disguise scrollbar behaviour if it affects the page width.