it's only helpful if your site is quality and people will actually use your rss feeds, otherwise there's no point.
Yes, it should be helpful to your website as long as you are submitting them to the right directories not the spam ones. Icerocket, Feedage and Alltop are some good ones that you should submit to.
Is people still use RSS ? that good question i think , i searching big site like BBC , NY times and etc , i still see them put their RSS link.
Yes of course it is useful. If your website publishes articles and content on regular basis then it's a great idea to have all those articles and news at one place and if some likes your articles they can easily subscribe to it. So in my opinion RSS feeds will always be beneficial for webmasters and readers both.
When doing niche research I've come across many websites with a TON of traffic from various RSS readers like Feedly. If you have a content based website, with a lot of high value content people will want to engage with long term, then yes, rss if valuable. Also, it's so easy to setup, might as well provide one more method of content consumption.
