Not the point.
This is a Unix-like system. It should be possible to plug in an external editor, whether it's vi-like, emacs-like, kedit, nano, or whatever. This is especially true when the user has a preferred set of keybindings and "feel" to an editor.
Actually, I'd claim that it would be highly reasonable for an e-mail client to open an external HTML editor if the user really wanted to send HTML mail. (Yes, there are occasions when it's justified). Depending on the situation (which should include knowing which MUA recipients use), this might allow for specialised editors that make it easy to send a particular type of very rich hypertext.
No-one who understands the power of pipelines complains that you shouldn't be able to pipe ls through sed "because sed is a strange-baked cake that has been superceded by Perl". The important thing is being able to plug in an editor, not what that editor is.
(And yes, I do use vi and friends a lot...)
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