Should we try to match the differential impedance of a CML pair, and if so, why?
My question is, why should we try to impedance match a differential
  pair connecting CML devices? Does differential impedance even mean
  anything since the conductors will never carry equal and opposite
  signals?


You have to dislocate the DC scenario from the superimposed AC signalling. Numerically the current may not reverse but, as far as sending data down a balanced pair of wires you need a balanced load.


  Would it not be better to treat each conductor individually and match
  them to the single ended impedance of the devices?


However, twisted pair is a tad notorious because it tends to require a balanced load that is not only transverse but longitudanal as well so, your 2nd quote does carry a certain amount of decent weight of truth.

In my experience with +100Mbps data signalling I've tended to fall towards treating a piece of twisted pair as two individual wires and used 2 balanced load impedances connected to screen.