Amongst other things, this means you cannot properly toggle Rmail's mime
decomposition off.

Using attached example mime message (uncompressed):

emacs -Q
C-u M-x rmail RET /path/to/mime RET
M-x rmail-mime RET

This still leaves a button at the end of the buffer:
  [2:image/png Show Save:topbanner.png (6kB)]

Compare with:

emacs -Q --eval '(setq rmail-enable-mime nil)'
C-u M-x rmail RET /path/to/mime RET