I don't know - in Star Trek I always want McCoy in my Kirk and Spock pairings, and he *is*, I think, often put in that role with one or both of them. (Especially Kirk/McCoy in reboot, but it shows up in TOS slash and OT3, too. And in canon and paracanon... there's a fairly classic Spock/McCoy fic where they get thrown back in time and Spock and McCoy *are* Holmes and Watson.)

Since I so totally a McCoy girl, I am not sure about K/S by themselves as a pairing - (though often even in K/S fic McCoy is still in the background, sidekicking/wrangling for them both.) I think it *is* a different kind of pairing; the dichotomies that get played up with them aren't really around the same axes, so while there are sometimes stories and characterizations that pull on the Holmes/Watson genius-wrangler type relationship, it varies which one goes in which role.