Bit of a boring request but this issue exists in current Magento but I see this as a problem going forward also.
This newsletter_subscriber table doesnt store the created date for a subscriber or the source e.g. newsletter box or during checkout. With more and more laws coming into play with proving when a person subscribed and the reason why/how they subscribed this will become even more of a problem. A recent Canadian Law CASL which actually covers anyone emailing a recipient located within Canada (so could be anyone) the fine is hefty and proof is key (http://www.dotmailer.com/blog/what-marketers-need-to-know-about-casl-canadian-anti-spam-legislation/...).
So a created date column and source to specify on your site where they subscribed would solve this.