I always ignore those comments, they usually get caught in my spam filter that I have set so they usually never actually appear on my videos.. But yeah, sub for sub/begging for subs is very common these days... people do it on so many videos its crazy
I don't think its rude to delete those kinds of comments because they're sometimes just copy and paste basic compliments like I got one the other day saying something along the lines of 'I love your content keep making videos! they're really good! Check out my channel to and subscribe
' very generic, very basic, probably copy/paste on hundreds of videos..