Banelings only cost half of supply. By not cost supply, I mean additional supply. Every other morph coasts additional supply. Zerg may like to remax, but having a spell that specifically kills a unit is stupid. Zerg always wants to retain their units especially high cost units.
You would use it on the lings because too many, your ling might not survive long enough to do it, and anything other than a roach is so expensive to justify killing it after 15 seconds. Hydras are expensive, ravagers, lurkers, mutas, BL etc. There is no unit in the game for zerg that they would actually use it on.