+1 for Featured Image in subscription emails!
I am not actually sure why there were “heated discussions”. The featured image is as much a part of an article as the text. I think the only case might be where someone is using a poorly designed or legacy theme that doesn’t include the featured image, so they manually add it to each post. That would result in the image showing up twice in the email, so maybe an override option for those using bad themes is in order.