Allowing more or less arbitrary attachments was a real convenience.
But allowing such stuff to serve as the message proper was
dubious at best. Not only did it require recipients to obtain
special software to read some messages; it also posed a
security threat.
I still use mailx precisely because it will only display plain text.
With active text such as HTML, it is all too easy to mistakenly
brush over a phishing link. Outfits like Constant Contact do their
nonprofit clients a disservice by sending stuff that I won't even
peek at. And it's an annoying chore when companies I actually want
to deal with send receipts and the like in (godawful) HTML only.
Doug