Although I don't use XDM, I do occasionally use 8-bit displays.  In one case,
I have used it with an old video card---a card which is great for 2D, and
which doesn't need to be replaced until the mythic day that we get DRI
support in NetBSD.  The card is a little faster at some things, and due to
limited memory (4MB) supports higher resolutions in 8-bit mode than in
16- and 24-bit modes.

8-bit isn't completely useless, even if you can get a usable 24-bit display.

And if the XDM image can be put into 8-bit without compromise, who is
hurt by doing that?  (^&

(This is all abstract for me, though.  Again, I don't use XDM...)

-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/