When the setting is enabled, text is parsed in the browser to find its direction. This will have some effect on the site’s performance. It would be good to be able to only enable the setting for users who need it. The setting was designed to help RTL sites that have a lot of code in their posts. For that case, it makes sense for it to be enabled for all users.
I think that for the case of a user’s locale having a different direction than the site’s default locale, it could make sense to automatically parse post content for its text direction.