There are several things to keep in mind when using WYSIWYG for transactional e-mails:
- Sending out HTML e-mails which can be viewed in a nice way in multiple e-mail clients is not always fun & easy: There are some "specialities" from mail client to mail client that need to be taken care of
- It could mess up your e-mail design and won't benefit some special adaptions as mentioned above
- You're limited to the WYSIWYG functions in your e-mail template. When I think of adding conditions like
{{depend object.function()}} .... {{depend}}
this can't be inserted with an WYSIWYG anyway.
- It will not work with responsive e-mail templates as you would not be able to edit reponsive e-mails with a WYSIWYG editor
- The WYSIWYG editor will try to interpret layout handles
{{var items_html}}
as widget instances which can cause problems (thanks to @SanderMangel for pointing that out).
I would not say that's impossible, if you find the ultimate WYSIWYG editor for transactional e-mails that does not cause problems, tell us :-)