Whatever email client you're using likely has easier/better support for bold/italics or other forms of text styling very easily, with shortcut keys. It probably also supports numbered/bullet lists automatically by converting lines that start with 1., 2., ... or */- characters, too.
If you think about it, there's not a lot more features of Markdown remaining. The email client also likely allows easier ways to insert pictures and create links, too.
This leaves us with:
- Tables: I suspect anyone actually writes tables in markdown, also the mail editors probably support this better.
- Math equations: I suspect anyone actually uses this except academics.
- Code blocks with syntax highlighting: this seems to be the only useful feature of the extension.
I also realized the extension does not work on Google Docs, where I was mainly hoping to use the code formatting feature.