In general, the utility of the top level classification of MIME types
is limited.
However, the gross characteristics of image/ audio/ text/ video/ are
useful as far as classification of remote device networks (can you
print it, can you hear it, can you read it as ascii, can you view
moving devices).
Certainly, if we stick to that general justification of top level
types, there is no place at all for 'chemical/'. Or 'structured/'.
I'd like to see if we can avoid having two types registered that
differ only by the top-level type, e.g., don't register image/frob and
application/frob; in particular, this will allow reorganization of the
types into categories, or even making the categorization optional.
There are arguments that at some point we might want application/html
(this is exactly html, except that it allows charset parameters that
aren't allowed by text/*) or image/postscript (this is postscript
that only contains images).