There are arguments for not using HTML for email at all or at least for providing a plain text alternative. One is that images need to be actually embedded in the email itself if you want them to be seen as many people rightfully disable external image links in emails for security reasons.
Another reason is that many people define all HTML emails as spam and so will never see your email at all unless you allow them an option to receive it as plain text.
Yet another reason is that HTML and emails were never intended to go together. That HTML works in emails at all dates back to the HTML 3.2 days of 1997 where the email programs of the time were modified to accept HTML 3.2 format for emails. No such modification has been made since and HTML 3.2 is still the latest version that many email programs accept. The only concession they have to more modern coding is that some of them will accept style attributes embedded in the HTML to apply some minimal CSS to the email content.
One difficulty that people have when asking for help with their HTML emails is that they neglect to specify that the HTML is for an email and therefore needs to be written as HTML 3.2 and need to follow the coding standard that became obsolete for web pages back in December 1997. By not mentioning this they almost guarantee that any answers they get will tell them they have everything coded wrong and will present them with more modern alternatives that work fine for the web but which will definitely not work in their email. Only by clearly identifying that the HTML is intended for an email and therefore needs to use HTML 3.2 and not 4 or 5 and without any external CSS or image references are they likely to get a usable answer.
The thing to remember with HTML emails is that they are no more advanced than web browsers were 20 years ago and none of the new features introduced over that period are likely to work in the email (at least not for many of the recipients). In most cases if a proper HTML page is needed a simple text email with a link to the page will give a better result. I receive lots of HTML emails where the sender hasn't a clue what they are doing and the email is not readable at all - in some cases they have included a 'view this email in a browser' link and you can see what the email looks like there so why bother with all the extra unusable junk that makes the email way bigger for no benefit.
This article written by Stephen Chapman, Felgall Pty Ltd.