extend t inheritance

[Ingo Lohmar]

I am messing around with the face :extend attribute and I am wondering
about sth.  The elisp manual says

     The value should be ‘t’ to display the empty space between end of
     line and edge of the window using this face, or ‘nil’ to not use
     this face for the space between the end of the line and the edge of
     the window.  When Emacs merges several faces for displaying the
     empty space beyond end of line, only those faces with ‘:extend’
     non-‘nil’ will be merged.

I briefly read the original discussion of this feature, but did not find
a reason for this peculiar behavior when it comes to inheritance; if
there was a reason given, could someone point me to it?

Background: I manage all faces without themes on my own, and I rely
heavily on inheritance.  As it stands now, IIUC (and according to some
tests), I have to specify :extend t on ~30 faces (mostly diff, ediff,
smerge, magit-ediff and -blame etc) that all derive from 3 or 4 base
faces.  It seems to me that this renders the whole point of inheritance
moot.

My natural reaction would have been to specify :extend t on some base
faces, and for faces that inherit from them, explicitly say :extend nil
only for those faces for which I do not want that extension.  Isn't that
at least as flexible, and more in the spirit of inheriting attributes?