On the one hand, the 'subscribe' pop-ups are intrusive, and I am reluctant to clutter my inbox with notifications, but on the other hand they provide a subliminal advertisement that the website is a source of material that is actually worth signing up for, along with implicit peer pressure from the implication, justified or not, that there is already a community of people who have signed up. It is a psychological nudge, and you seem to be saying that the benefits outweigh the downsides?