character entities

6.2.1 lists some non marking entities, and introduces them with the

   Some character entities, although important for the quality of print
   rendering do not directly have glyph marks that correspond. 

But are they all really _character entities_ ie things that have
definitions such that they can be used in elements with PCDATA


For a style sheet I can define that to be the empty string
and it can be used in mo as <mo>&InvisibleTimes;</mo>

but are the spacing entities really intended to be used inside
such token elements, or outside.

I am tempted to define 

&MediumSpace;  to be <mspace width="something">

but then &MediumSpace; could only be used where mspace could
be used (ie not in token elements).

In that case I have the alternative of defining &MediumSpace;
to be the unicode character 2005 but then &MediumSpace; can _only_
be used for spacing in token elements, and not the place one normally
needs to adjust the spacing, ie between elements.

The problem is more acute for things like 


where as far as I can see there is no unicode equivalent so I don't
think I can map it to any character, so it appears the only sensible
alternative is to map it to <mspace width="-something"> but then,
it could not be used in  character elements, which I think is the usage
implied by the recommendation?

Is it really the intention that these spacing commands are used as
characters rather than <mspace> elements?