No, I feel that html messaging should be allowed as that's the current
standard for sending mails.
There have been quite a few mails regarding this topic so I am not sure if
this suggestion has already been given....
If mailman could take the html messages and send them as an attachment.
People with text based email clients can also view these mails and ignore
the attachments if required. Others who wish to view the HTML messages can
view the attachment in a web browser.
I feel this is a better alternative to filtering the HTML tags in a message.