We want users to unsubscribe to messages they don’t want; we don’t want them to mark them as spam and hurt the reputation of the sender. We have seen by implementing this unsubscribe affordance in the UI that spam marks go down and in some cases are being reduced by 30 to 40%.

They're less likely to go into the "Promotions" tab in Gmail (used by ~16% of all email users), for the same reasons above. From my testing, the plain emails typically end up in the Updates tab and some times even in the primary tab. Of course, the text in the email also affects this.

You can use a free spam checker to validate this by testing plain and designed emails.