I too vote for an administrator option to specify TEXT or HTML.
I'm less concerned about the client-side arguments as I am about my ability as an administrator to specify.
It's not a huge deal to me, but I do have an automatic process that processes these emails to generate an external email list which is used by MailEnable; I then export the email to my plain text database.
The HTML in there just makes my plain-text database of registrations a bit uglier and a lot bigger.
