I can imagine a case when datum reference in a single segment positional FCF for pattern of features is not needed at all - as Dave just said, we can consider pattern of 4 spheres where, for the functional reasons, the spacing within the pattern is more important than the location of the pattern itself. And that's fine for me, because spheres are very specific features which do not have any orientation defined. So the mating part with 4 spherical hollows in it will fit as long as a spacing within the pattern is sufficient.
But if we consider mating of two parts like: a flat plate with a pattern of 4 cylindrical holes and similar plate with 4 pins, and we do not use any datums for position of these patterns, we will not be controlling perpendicularity of holes/pins in relation to mating surfaces at all. So there might be a case when pattern of holes is tilted in one direction and the pattern of pins in opposite direction, and then the alignment is not possible.