Unfortunately, no they aren't common to all messages.
The Activate Email message has a nice layout of potential sales-based features that the user is enticed with to reinforce their onboarding subscription.
The Welcome Email message has a different layout for walking people through some of the content on our website to get the most benefits from what we offer.
The Goodbye Email message has a custom layout with an exclusive offer to help with customer retention.
--
I think that the option to use only an HTML editor would be useful here, considering that many people could be using their own HTML templates from different sources to customize their content like we do.