I have a video on this subject, and have talked to people at YouTube about it. Basically it adds more data which is good. It helps really point to places like on Google highlights if people are searching for a specific thing. They're not going to make a video blow up, but the more data you can give YouTube the better ESPECIALLY for search based content. If you have the time add them, but they're not the most important thing. That is the content. I'd say if you can add them, do it. If you can't focus on better content.
They also play a humanitarian side where you're helping more viewers enjoy your content. Overall, you gain more than you lose. I agree with what
@Stanley Orchard and
@Dr Pixel Plays says and look at your analytics. See if people are using them and where.
I use them on all my videos, and will continue to moving forward. Have for years (In English at least) can't speak every language haha...