It may just be me, but I find the outcome of message composition unpredictable, whether in text or HTML mode.
In fact, I would be interested to know why both methods are available - I would prefer (and other email clients are able to provide) a single message composition area that just works, including formatting and bulleted / numbered lists.
I haven’t got any examples to show at the moment but could create examples if required.
Do you mean how the message looks at the receiving end, or how it looks while composing the message?
It is useful to have both a plain text and HTML editor available so you can choose if you want formatting or not. Email messages often contain one or the other or very commonly both formats. A message containing both gives the recipient a choice as to whether they want to see the plain text or HTML versions.
In fact, an advanced feature would be to have an editor that gives you the ability to compose separate messages for HTML and plain text modes so that you have maximum control over what your recipient sees depending on whether they choose to view the message in HTML or plain text format. At Runbox we’ve done this sometimes with service messages as we wanted to make sure that recipients who use either method of viewing our message see something that is how we intended it to be. I digress though…