I think the relevant part of the doc is
% Particularly, a number of broken or ugly design sizes are no
% longer used, the look of the bold sans serif typeface at large
% sizes is considerably improved, and mismatches between the text
% fonts and the corresponding math fonts are avoided. As a side
% effect, PostScript and PDF documents may become smaller, because
% fewer fonts need to be embedded.
So while like any design choice it could be questioned, I think the
fact that it does not use all the provided design-size fonts is
certainly deliberate. (I assume the above is Walter Schmidt's original
documentation text, but I didn't check just now.)
David