my stance is this:

if a theme is made around the wallpaper, then it should be included. also, if the wallpaper was made specifically to complement the theme, it shoudl be included.

however, if the author just included the wall because they liked it at the time (with permission from the author of course), then that is just stupid. they should link to where the wall was originally found in the readme or in the *.thm since it isn't integral to the theme and just ups the d/l size a good chunk.