We *should* have is face inheritance.  Then 'bold would inherit
font family from 'default.  Unfortunately, in the old X bitmap
font model faces like bold and italic had to be provided as
separate font files, and many fonts didn't provide them.  So
finding a bold font for any given font often required search
beyond merely changing the weight parameter.  Since code
needed to be called dynamically, it kind of needs to be done
explicitly, and it isn't always done.