Should my scripts send email in text/plain instead of text/html?

I can do so much more in an em开发者_StackOverflow中文版ail with a text/html content type.

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Not all e-mail authoring tools support HTML markup. They should not send text/html if that is not what it was intended to be.

MIME types such as text/plain and text/html go beyond e-mails. They describe how content should be interpreted (by web browsers, mail clients, etc). If there's no markup on the data provided by the author (i.e., it is intended to be rendered as plain text), then text/plain is very useful for that (has been and always will be).

EDIT: You have changed your question a few times since I wrote this answer. To answer your latest version of the question, you should support text/plain if you want to allow your users the option of sending mail as plain text. Some people (such as me) prefer it. We don't like the un-necessary extra traffic and we don't like clients attempting to render it in any way but plain text (to preserve text-based formatting, like line breaks, spaces, tabs, etc).