Server-generated mail-outs could use this technique to determine how well targeted the mailing is. Perhaps you could put a subscribe/unsubscribe button on the form and tie that to a user ID. That would get round one of the major difficulties of unsubscribing people, which is to do with the mutations that mail addresses undergo. Periodically, a mail server may move or a domain name may change, and this means that unsubscribing from the new address does not locate the subscription record that was made under an old address. Passing an ID back and forth solves this major headache.
Just because something is possible does not mean it is advisable or good to do.
On the other hand, within the confines of a closely controlled Intranet or workgroup, this could find many useful applications. Just so long as you know where the mails came from and you can absolutely trust that they have not been compromised.
Personally, I'd recommend that you turn it off. I've not yet been convinced of the need to support HTML in e-mail correspondence other than for demonstrating how hip and trendy I can be.