Here’s a little rant I’ve been meaning to get out for a while.
The whole point of the multipart/alternative data type is progressive enhancement. A client is free to select from the alternatives presented and render as best it can, with an option for manual selection (that is, as long as you don’t use Outlook which doesn’t believe in such things). This applies to the common text/plain > text/html combo as much as it would to text/plain > image/jpeg, or perhaps application/pdf > application/vnd.sun.xml.writer. Now if they restricted their comments to text/html only, I might have some sympathy, as that’s just shoddy behaviour on the part of the sender. However, they usually prefer to throw out the baby with the bath water.
To conclude: MIME is a wonderful thing; some people use it badly; get over it.