For me personally, I'd put it at a 5 or 6. Once I return to commuting to work, closed captions would allow me to watch a lesson while on the train or bus without having to use headphones, but personally I like listening to the audio.
For people that are deaf/hard of hearing, I'd put it at a 11/10. SVS could reach new audiences that do not currently consider your classes or subscriptions because they are video-based and lack captions. If you caption in different languages this could also open the classes up to people who's english may not be very strong.