Your use of the word "still" is a little misleading. IE _was_ the default email rendering engine until 2007 when Microsoft made the inexplicable decision to move to Word 2007 instead.
My theory is still that it was about using Word as a component to write replies. However, if they delayed the HTML->Word conversion to that point, everyone would complain to MS "Reply is broken!"
By using Word as a viewer the email is broken right on delivery and users now complain "The email you sent me is broken!"
