“It is frustrating to receive an email that is broken or can’t be viewed in some way, hence why offering both options helps ensure everyone receives your messages. Not offering multiple format options for your email could affect your unsubscribe rate substantially.”
Depending on the email client, browser and mobile device, finding a universal email format can be a challenge. Plain text and HTML formatted emails each have useful applications but maybe not for every list segment or at every time of day. Test formatting styles to see what works best for readers at different times of the day.