For WHM one could free up two slots to use for new/lost abilities/spells on the hot-/cross-bar through new traits by letting Medica I change into Afflatus Rapture, and Cure I into Afflatus Solace, while lilies are available.

I can't really think of a situation at higher levels (with all our mp regen and size of pulls) where one use Cure I or Medica I instead of Tetragrammaton/Afflatus Solace or Rapture, as AS and AR cost no mp and are instant, and if there are no abilities to burst heal, then one would most likely use Cure II/Medica II. There used to be a difference in cast-time of Medica I/II if I recall correctly, but that's no longer the case, and the difference in mp use is quite small. Cure I is 1s faster than Cure II, but in my opinion it's not certain the heal from Cure I would be powerful enough to save anyone with the 1s spared.