I tried doing it, I actually thought I'd do it so deaf people might be able to at least see what I'm saying commentary wise, but omg... I did like 1-2mins and gave up because I wasn't seeing progress. If it was a scripted video, maybe then it'd be okay as you could pretty much copy and paste it in (even then, it's a task and a half), but stuff done on the fly that can go up to 30mins +? I think there's software that can add captions easier than youtubes system though, there may even be a way to do it via Premiere/AE but I haven't looked into that. The thing you have to ask yourself is: would hearing impared people be likely to look up your video? In my particular case I don't overly think so as they can get gameplay from literally any YouTuber as it wouldn't be a difference, we'd all be the same to them, especially if they're deaf.