I can only relay my specific experience. I didn't buy the QS8s right away and was going to use my old trusty HK bookshelves for surrounds.
I adjusted and readjusted their angles but they were just to directional or something as they just seemed lacking despite their great performance as fronts in my old much smaller house.
I saved up for QS8s and was blown away by the difference in surround effects and I preferred them for music as well. To phrase it in less than professional terms, the room just semed more filled up with music. Admittedly, I didn't swap them in and out to compare.
