Some details:

eth and thorn are used in differential geometry as mathematical symbols.
There is an apparent shortage of d-like symbols in this field.

Arrangement of the font tables (and this argument goes really LaTeXy)

One could have essentially two points of view on the table arrangement:

a) Providing optimal kerning for a core of 256 glyphs. This would lead to
   a mixture of upright and math italic glyphs in one table together with
   basic braces.

b) Logical markup (more on the LaTeX path but sacrificing optimal kerning):

   Make \mathrm{\pi} work (like \mathrm{\Pi} etc. already do work). This
   means sorting out the glyphs according to their fontshapes in different
   mathematical familes.

   Note that the question after \mathbf{\pi} is an FAQ on comp.text.tex and
   it is tedious to tell the users again and again to use boldsymbol on
   that, whereas \mathbf{\Pi} works fine.

--J"org Knappen