Stephen,
That's an interesting query you pose. From my perspective, there are so many QSAs missing their KSA that I no longer take that into consideration when purchasing a QSA. The occasional reunite is wonderful to achieve but I am normally content with the QSA which, of the two, is the more interesting for me.
When I see single KSA's on lists and catalogues I do check them to see if any are 'mine' so to speak and, if not reunites, I guess people buy them for their rank, unit or if they are single clasp examples?
Regards
David