This article misses the point of why emails were made interactive in the first place - to satisfy the demands of marketers. 90% of the emails you get are marketing-related. If you get an email that says "4 days left for our holiday sale", that counter will need to be updated if the email is not read on the same day. It's a small, maybe frivolous-sounding use case, but a lot of feature bloat starts out this way.