This is a very useful feature which makes visually more noticeable
all potentially dangerous control characters or other special glyphs
which doesn't correspond to a character.
Ok, I can see that that could be useful. But implementing it
by setting up a default display table is a kludge. The right way
is to change the code that implements the standard display conventions,
to make it use the desired face when it generates these glyphs.
lot of things in Emacs have default faces on by default. Why should these
strange multiglyph characters not also?
Actually I just saw it in action in a buffer that was outline-minor-mode'd
(an elisp buffer where every function is folded on a single line) and the
grey background face on the "..." looks really ugly.
I don't mind it on ^L and \201 kind of things (which aren't pretty anyway
and do deserve some kind of warning), but I think that on the ellipsis
glyph, it's a sin.