Wow, this does sound like a good idea for all those marketing people omitting the plain-text variant for their HTML mail.
It usually goes accompanied by text saying 'It seems you are not using a modern e-mail client to view this message, you can view it here --> link to marketing agency.
One of my bugbears is agency created emails that are sent direct from the agency servers or include agency links for tracking. My relationship is with your business, keep them out of it (or at least out back where I can't see them).
It usually goes accompanied by text saying 'It seems you are not using a modern e-mail client to view this message, you can view it here --> link to marketing agency.