As I said, if you were crossing similar hunting breeds out of proven stock then you'd be breeding for that instinct. But nobody is really doing that -- your mixed breed dog has not been deliberately bred for any particular purpose -- so we're in the realm of theory here.
By the same token, if you tried to train two dozen mixed breed dogs and two dozen field-bred springers as guide dogs for the blind, you'd probably have better success with the mixed breed dogs for precisely the same reasons.