I just always have subs on. There are shows which are downright unwatchable without subs (e.g. True Detective). I'm not a native speaker, but I'm fluent.
If I could, I would have the entire script as well, like Script to Screen does: https://www.facebook.com/pg/scripttoscreen.film/videos
One explanation might be if you're watching a 5.1 surround sound movie and playing it through your monitor's built-in speakers which can't handle the extra channels.
I work they use fewer words....it's unfortunate when it spoils a delivery several seconds before it happens.
If it's a comedy, there's not going to be any sort of quiet sounds, so I keep the subs off. If it's an action that usually has gunshots way louder than dialogue, I'm going to adjust the volume to those gunshots and turn the subs on.