I think this is entirely dependent on the context of that particular E-mail. Email has evolved quite a lot.

A good example is a newsletter from say, Quora or Medium; Newsletters usually have links to a story or a news feed article. If this was done using plain text, the link would be one long mashup of characters, because they usually include an authentication token or something like that. In this case, using an html link or button is clearly the better option.

HTML is like structured text for web pages.