If you get comments on YOUR videos, Youtube (+ Google) ranks your video better because it is getting "attention" If you comment on other peoples' videos, you might get some traffic back to YOUR videos or channel... but don't use it to link to your sites as this isn't worth much.
Take the efforts and time and delete them! Most spammers don't like wasting time and won't waste more time spamming on your blog. If you can find a way to prevent automatic posts so that sure will help a lot. Spammers are the ones who make it so hard for those people trying to build backlinks by using their time and to post serious and blog post related quality comments and spammers are the reason for so many blogs to be nofollow. Good Luck and much success for your blog verdecove