My rear speakers are about 7 to 8 feet behind my listening position with no back wall to speak of, the entire room is over 1000 sq ft. If I had a narrow walled in space perhaps direct radiating speakers would work just as well.
With the new 7.1 sound formats on several Blu-ray movies out now, if you have the room rear speakers Imo will be more than justified.

A computer once beat me at chess, but it was no match for me at kick boxing.